Abstract - the UWA Profiles and Research Repository · Mr Tom Fox for providing certified “Jasper Lake” seed potatoes cv. Atlantic and Granola. My parents, bapak Ketut Suwinda

Abstract

Potato (Solanum tuberosum L.) varieties Atlantic and Granola are widely grown in

Indonesia. The optimal method of cultivation in the tropics, due to the susceptibility of

cut seed for disease, is by small (20 to 55 g) whole seed potatoes. However, the variety

Atlantic produces mostly large tubers, which are not suitable for planting as whole

seeds. Although Granola produces a reasonable proportion of small tubers it still

produces a few in the larger size grades and there is no fresh market in Western

Australia for the larger tubers for this variety. The aim of this study was to develop

methods to be used in Western Australia that improve the yield of small seed potatoes

for export to Indonesia. The influence of seed-potato storage duration (at 4oC) on

subsequent stem growth was assessed after 30 days growth in a glasshouse (22oC/18oC,

day/night). Seed potato storage for 22 – 28 (Atlantic) and 24 – 30 (Granola) weeks

resulted in development of higher numbers of stems. A series of field experiment were

designed to increase yield of small tubers. Apical sprout removal in Granola, but not

Atlantic, increased the number of stems (by 27%), yield of 20-55 g potato (by 32%) and

total yield (by 17%). Application of herbicide (paraquat + diquat) at low concentration

during early tuber initiation decreased total yield in Atlantic (by 14%) and Granola (by

16%). Treating whole seed potatoes with carvone vapor two weeks before planting had

no influence on stem or tuber number in both Atlantic and Granola but in Atlantic only,

the total yield was reduced by 12%. Spraying plants with paclobutrazol during early

tuber initiation inconsistently influenced tuber number and yield between the two

varieties and two experiments. The influence of gibberellic acid (GA3) on stem number,

total tuber number, yield of 20-55 g tubers and total yield was investigated by dipping

seed pieces in a GA3 solution (20 mg/L) two days prior to planting. In Atlantic, GA3

treatment increased stem number (by 147%), total tuber number (by 75%) and yield of

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20-55 g tubers (by 330%) without influencing total yield. In Granola, GA3 treatment

increased stem number (by 50%), total tuber number (by 15%), yield of 20-55 g tubers

(by 21%) and total yield (by 10%) The influence of gibberellic acid application (20

mg/L) to seed pieces before planting increased the number of small tubers through

increased stem number. The shift toward a greater proportion of small tubers, without

reducing total yield, had a greater influence in Atlantic than that in Granola. Treatment

of GA3 and paclobutrazol together decreased total yield compared to that of GA3 alone.

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Publication

Arpiwi NL, Plummer JA, McPharlin IR (2003) Gibberellic acid increases yield of small

round seed potatoes (Solanum tuberosom L. cv. Atlantic and Granola). Poster presented at

the ComBio 2003 conference in Melbourne, Australia, September 28 – October 2.

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Acknowledgement

My sincere thanks to the following: My supervisors, Associate Professor Julie Plummer and Dr Ian McPharlin from Department of Agriculture Western Australia for their ongoing supervision, support, wise counsel, constructive criticism and encouragement through the course of this study. AusAID for my Scholarship and my AusAID Liaison Officer, Mrs Rhonda Haskell for her warm welcome at any time I needed her. Also for her great encouragement and help in solving my study and personal problems. I am very grateful for the financial support for the field experiment in Manjimup which was from the project “Market product development for export seed potatoes to Indonesia” funded by The Regional Assistance Program (RAP) through the South West Development Commission (SWDC) Bunbury. I also very greatful for financial support for field work at Shenton Park (Perth) which was from Horticulture Australia Ltd, project PT 02014 “Sustainable agronomy packages for export potatoes”. Without these funds this research would not have been possible. Staff in Department of Agriculture Western Australia for help in taking potato plant samples. I would especially like to thank Tony Shimmin for his invaluable technical assistance during all field experiments, friendship and humour, from him I learnt ‘toing’ and ‘froing’. Gavin d’Adhemar for organizing and operating the planter, harvester and hilling machines. Staff in the Manjimup Horticultural Research Station, Department of Agriculture Western Australia, particularly John Doust and Rachel Lancaster for their technical assistance during field experiments. Staff in Department of Agriculture Western Australia at the Bunbury District office especially Jeff Mortimore for help in taking plant samples in the field. Also I want to thank Kuswardiyanto for drawing a map of potato growing areas in Western Australia. I would like to thank Phil Ross for organizing and sending seed potatoes from Manjimup to the UWA. Mr Mike Blair, manager of the University of Western Australia Shenton Park Field Station for providing me with a site for potato planting and doing the irrigation. Mike Shane for careful editing of the final draft of this thesis. Mr Greg Cawthray for technical assistance in chlorophyll analysis, taking leaf samples in the field, carbohydrate analysis and HPLC. Dr Anh Van-Pham for helping me with data analysis. Administrative staff in School of Plant Biology, Sandra, Rhonda, Jeremy and Paul for their practical help.

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Western Potato for providing data about yield of potato in Western Australia. Mr Tom Fox for providing certified “Jasper Lake” seed potatoes cv. Atlantic and Granola. My parents, bapak Ketut Suwinda and ibu Made Kasning for moral and spiritual support and a peaceful life. I am very proud of them. I value their encouragement and guidance throughout my life. The biggest support came from my one love Suma for being an ideal husband. I greatly appreciate and thank him for his incredible personal sacrifice, patience, forgiveness, and support. Also for his help in collecting data in the field, in the glass house and grinding my tuber samples. Thanks also for your invaluable time in looking after our lovely children, which made it possible for me to study. My sons, Amartya Paramahamsa who always says ‘mum, are you going to finish your thesis?’ that was the only sentence he said anytime I left him to go to the campus. Ozzy Dwijay Wirawan, who was born in Perth during my study. Both of you are great sources of inspiration. Ade and Mala, exchange students from Udayana University Bali who did work experience at the Department of Agriculture Western Australia for their help in fertilizing, sampling and harvesting My officemates, Anna, Andreas, Lisa, Katherine and Maratree who were very helpful and provided a nice study environment. Especially for Anna who was very kind and helpful in many aspects particularly with operating computers. My Indonesian friends, Asta, Anne, Toni, Oon and Bingah for cutting 200 kg of seed potatoes. And most of all, to God Almighty who made all this possible.

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Table of contents Page Abstract ................................................................................................................ i

Publication ........................................................................................................... iii

Acknowledgement ............................................................................................... iv

Table of contents ................................................................................................. vi

List of figures ....................................................................................................... viii

List of tables ........................................................................................................ ix Abbreviations ..................................................................................................... xi

Chapter 1 General Introduction ....................................................................... 1

Chapter 2 Literature Review ............................................................................ 11

2.1. Potato morphology and anatomy ........................................... 11

2.2. Seed potatoes ......................................................................... 14

2.3. Stolonization ......................................................................... 16

2.4. Tuber initiation ...................................................................... 18

2.5. Dormancy .............................................................................. 19

2.6. Apical dominance .................................................................. 22

2.6.1. Manipulation of apical dominance .............................. 23

2.7. Physiological age ................................................................... 24

2.8. Factors influencing potato tuberization ................................. 29

2.8.1. Photoperiod ................................................................. 29

2.8.2. Temperature ................................................................. 31

2.8.3. Irrigation ...................................................................... 33

2.8.4. Soil properties .............................................................. 34

2.8.5. Plant growth regulators ............................................... 35

2.8.5.1. Endogenous gibberellins ................................. 36

2.8.5.2. Exogenous gibberellins ................................... 38

2.8.5.3. Abscisic acid ................................................... 42

2.8.5.4. Auxin ............................................................... 43

2.8.5.5. Cytokinins ....................................................... 43

2.8.5.6. Jasmonic acid .................................................. 43

2.8.5.7. Paclobutrazol ................................................... 44

2.9. Determining time of tuber initiation ...................................... 47

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2.10. Factors influencing tuber size distribution .......................... 47

2.10.1. Number of plants per unit area ................................ 48

2.10.2. Number of stems ..................................................... 48

2.10.3. Number of tubers ..................................................... 48

2.11. Conclusion .......................................................................... 50

Chapter 3 Manipulation of apical dominance by chemical and physical treatments leads to increase tuberization in potato (Solanum tuberosum L.) varieties Atlantic and Granola ......................... 52

3.1. Introduction ........................................................................... 52

3.2. Materials and methods ........................................................... 56

3.3. Results ................................................................................... 62

3.4. Discussion .............................................................................. 73

3.5. Conclusion ............................................................................. 78

3.6. Recommendation ................................................................... 79

Chapter 4 The use of gibberellic acid and paclobutrazol to increase the yield of small round seed potatoes (Solanum tuberosum L.) varieties Atlantic and Granola .................................................. 80

4.1. Introduction ........................................................................... 80

4.2. Materials and methods ........................................................... 83

4.3. Results ................................................................................... 91

4.4. Discussion .............................................................................. 109

4.5. Conclusion ............................................................................. 120

4.6. Recommendation ................................................................... 122

Chapter 5 General discussion ...................................................................... 123

References ........................................................................................................... 133

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List of figures

Figure 1.1 Potato growing areas in Indonesia 3Figure 1.2 Potato growing areas in Western Australia 8Figure 2.1 The morphology of potato plant 12Figure 2.2 Anatomical details of potato tuber 14Figure 2.3. Longitudal section of potato tubers showing internal

structure 14Figure 2.4 The morphology of stolon 15Figure 2.5 Characteristics of Atlantic and Granola potato tubers 17Figure 2.6 Stolon development. Stage A (elongating stolon tips), stage

B (swelling stolon tips), stage C (fully swollen stolons) and stage D (young tuber 1-2 cm across) 20

Figure 3.1 Potatoes grown in a glass house phytotron 57Figure 3.2 Potatoes grown in the field at Manjimup Horticultural

Research Institute 58Figure 3.3 Influence of storage durations (weeks) at 4oC on sprout

number 63Figure 3.4 Influence of storage duration (weeks) at 4oC on time of

emergence 64Figure 3.5 Influence of storage duration (weeks) at 4oC on stem number

using cut and whole seed potatoes 65Figure 3.6 Influence of storage duration (weeks) at 4oC on stem number

using cut and whole seed potatoes and four pieces of seeds of the same origin were combined 65

Figure 3.7 Influence of storage duration on plant height 66Figure 4.1 The relationship between SPAD-502 reading and extractable

chlorophyll a and b contents 98Figure 4.2 Influence of GA3 on yield (t/ha) of potato in different size

grades (G) and total yield at final harvest 118 DAP 108

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List of tables

Table 1.1 Export of seed potatoes from Western Australia to Asian countries from 1995 to 2000. 10

Table 3.1 Chemical and physical characteristics of top 15 cm of soil in fields for Experiment 2 and 3 grown in Manjimup 58

Table 3.2 Influence of apical shoot removal on time of first and complete emergence. 67

Table 3.3 Influence of carvone on the time of first and complete emergences 67Table 3.4 Influence of rates (250 and 500 mL/ha) and timing (early and late

tuber initiation) of paraquat + diquat applications on shoot dry weight (g) 82 DAP in Experiment 2. 68

Table 3.5 Influence of rates (250 and 500 mL/ha) and timing (early and late tuber initiation) of paraquat + diquat applications on shoot dry weight (g) 82 DAP in Experiment 2 on leaves chlorophyll (mg/g DW) content 84 DAP in Experiment 2. 69

Table 3.6 Influence of apical sprout removal on stem number per plant 82 DAP in Experiment 2. 69

Table 3.7 Influence of rates (250 and 500 mL/ha) and timing (early and late tuber initiation) of paraquat + diquat applications on tuber number per plant in different size grades and total tuber number at final harvest 146 DAP in Experiment 2. 70

Table 3.8 Influence of apical sprout removal on tuber number per plant in different size grades and total tuber number at final harvest 146 DAP in Experiment 2. 71

Table 3.9 Influence of carvone on tuber number per plant in different size grades and total tuber number at final harvest 147 DAP in Experiment 3 71

Table 3.10 Influence of rates (250 and 500 mL/ha) and timing (early and late tuber initiation) of paraquat + diquat applications on yield (t/ha) in different size grades and total yield at final harvest in Experiment 2. 72

Table 3.11 Influence of apical sprout removal on yield (t/ha) in different size grades and total yield at final harvest 146 DAP in Experiment 2. 73

Table 3.12 Influence of carvone on yield (t/ha) on yield in different size grades and total yield at final harvest 147 DAP in Experiment 3. 73

Table 4.1 Chemical and physical characteristics of top 15 cm of soil in fields for Experiment 1 (Manjimup) and 2 (Perth). 84

Table 4.2 Weather data during Experiments in Manjimup (October 2001- March 2002) and in Perth (August –December 2002). 84

Table 4.3 Influence of GA3 on the first and complete emergences 69 DAP in Experiment 2. 91

Table 4.4 Influence of GA3 on starch and total sugar content (mg/g DW) in developing potato tubers 69 DAP in Experiment 2 92

Table 4.5 Influence of GA3 and paclobutrazol on the length of the longest stolon (cm) 69 DAP in Experiment 2. 93

Table 4.6 Influence of paclobutrazol on stolon + root and shoot dry weights (g) at different harvest time (DAP) in Experiment 1. 93

Table 4.7 Influence of paclobutrazol on stolon + root and shoot dry weight (g) 69 DAP in Experiment 2. 94

Table 4.8 Influence of GA3 on stolon + root and shoot dry weight (g) 69 DAP in Experiment 2. 94

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Table 4.9 Influence of GA3 and paclobutrazol on internode length (mm) 69 DAP in Experiment 2. 95

Table 4.10 Influence of GA3 and paclobutrazol on plant height (cm) 69 DAP in Experiment 2. 96

Table 4.11 Influence of GA3 and paclobutrazol on leaf area (cm2) in Experiment 2. 97

Table 4.12 Influence of GA3 and paclobutrazol on chlorophyll content of leaves 76 DAP in Experiment 2. 97

Table 4.13 Influence of paclobutrazo on tuber number per plant at different harvest (DAP) in Experiment 1. 98

Table 4.14 Influence of paclobutrazol and GA3 on tuber number per plant 69 DAP in Experiment 2. 99

Table 4.15 Influence of paclobutrazol on tuber number per plant in different size grades (g) and total tuber number at final harvest 146 DAP in Experiment 1. 99

Table 4.16 Influence of paclobutrazol on tuber number per plant in different size grades (g) and total tuber number at final harvest 118 DAP in Experiment 2. 100

Table 4.17 Influence of GA3 on tuber number per plant in different size grades (g) and total tuber number at final harvest 118 DAP in Experiment 2. 100

Table 4.18 Influence of GA3 and paclobutrazol on tuber number per plant at final harvest 118 DAP in Experiment 2. 102

Table 4.19 Influence of paclobutrazol on yield (t/ha) in different size grades (g) and total yield at final harvest 146 DAP in Experiment 1. 102

Table 4.20 Influence of paclobutrazol on yield (t/ha) in different size grades (g) and total yield at final harvest 118 DAP in Experiment 2. 103

Table 4.21 Interaction of GA3 and paclobutrazol on total yield (t/ha) at final harvest 118 DAP in Experiment 2. 104

Table 4.22 The influence of paclobutrazol on yield in different sizee grades and total yield at final harvest 146 DAP in Experiment 1

105

Table 4.23 Influence of paclobutrazol on yield in different size grades and total yield in final harvest 118 DAP in Experiment 2 105

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Abbreviations

ASR apical sprout removal ABA abscisic acid CCC 2-chloroethyltrimethyl ammonium chloride DAP days after planting DAE days after emergence DW dry weight FW fresh weight GA gibberellic acid GA1 gibberellin A1GA3 gibberellin A3 G1 generation 1 G2 generation 2 E east S south IAA indole acetic acid JA jasmonic acid PAC paclobutrazol oC degrees Celcius cv. cultivated variety nm nanometer m meter m2 meter square mm millimeter cm centimeter L liter mL milliliter mg miligram g gram kg kilogram t tonne ha hectare t/ha tonne per hectare t/pa tonne per annum a.i. active ingredient h hour kPa kilopascal cb centibar max maximum min minimum % per cent MH maleic hydrazide IPC isoprophyl N-phenylcarbamate CIPC isopropyl N-(3chlorophenyl)carbamate DMN dimethylnaphthalene ® trade name lsd least significant difference N nitrogen P phosphorous K potassium

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Mg magnesium Mn manganese Zn zinc B boron Cu copper Mo molybdenum Pers.comm. personal communication HCl hydrochloric acid H2SO4 Sulphuric acid DI water deionised water v/v volume per volume AU absorbance unit

Chapter 1 General Introduction

Chapter 1

General Introduction

Potato is the fourth most important food crop in the world after rice, wheat and barley

(Fernie and Willmitzer 2001). Potato is a major horticultural crop in Australia and it is

becoming an important vegetable crop in Asian countries. The global areas dedicated to

potato production are shifting from developed to developing countries and from

temperate to tropical and subtropical zones (Horton and Anderson 1992; Jadhav and

Kadam 1998). The popularity of potatoes in developing countries is increasing due to

the high nutritive value of tubers and their relatively easy propagation (Fernie and

Willmitzer 2001).

Potato consumption in Indonesia is rising (Batt 1997) and has been precipitated by

changes in food habits (Horton and Anderson 1992) from traditional to western diets,

such as potato snack foods. For example, the consumption of french fries doubles

annually (Institute for Horticultural Development 1997). More traditionally however,

potato consumption increases during festivals, such as Lebaran day after the Moslem

month of fasting (Midmore 1992). In Indonesia, the government recommends potato

cultivation to its farmers as an important adjunct to crops like cabbage, shallots,

tomato and chilli. Development of vegetable production in Indonesia aims to fill the

demand for domestic food, improve diet and provide new jobs. Specifically, potato

cultivation will encourage industrial development, leading to increased exports and

decreased imports (Subijanto and Isbagyo 1988).

Potato harvest area and production in Indonesia has increased substantially over the last

40 years (i.e. from 10,000 ha in 1961 to 62,776 ha in 2001). Furthermore, potato

production has doubled every 10 years (i.e. from 60, 000 Mt in 1961 to 216,000 Mt in

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1981). Since 1981, potato production has increased to approximately 1,000,000 Mt in

2001 (Food and Agriculture Organization of United Nations 2002).

The main potato growing areas in Indonesia are Java (i.e. 62%) and Sumatra (i.e. 32%)

(Statistics Indonesia 2000). In Java, the potato planting areas include, West Java

(Pengalengan, Ciwedey, Garut, Lembang and Cipanas), Central Java (Dieng plateau)

and East Java (Malang, Pasuruan, Probolinggo) (Figure 1). In Sumatra, potatoes are

grown on the Karo plateau, Padang uplands, and highlands of Tapanuli, Benkoelan and

Brastagi. Small areas of cultivation are also found in Sulawesi, Nusa Tenggara, Maluku

and Irian Jaya (Bottema et al. 1991; Rhoades et al. 2001). The increase in potato

production is related to factors such as better per capita income, fast population growth,

urbanization, demand from hotels and processing industries and a boom in fast food

franchises (Institute for Horticultural Development 1997).

In Indonesia generally potatoes are grown twice a year, from September to December

(wet season crop) and from April to July (dry season crop). Generally, the wet-season

crops yield more than dry-season crops. The specific planting dates vary according to

location and cropping system. For example, at higher elevation, potatoes are planted in

April and October for harvest in August and March, respectively (Batt 1997; Rhoades et

al. 2001).

Many potato cultivars have been grown in Indonesia as consumer and grower

preferences changed. Before 1945 the main cultivars were Eigenheimer, Bevelander,

Voran, Profijt, Marita, Pimpernel and Bintje. Twenty five years later Désirée, Donata,

Cosima, Radosa, Patrones, Rapan, Thung, Katella, and Aquila. Today, Granola is the

main cultivar in Indonesia, accounting for approximately 80% of the cultivars planted

(Potts et al. 1992; Rhoades et al. 2001). This cultivar is very popular because it matures

early and has yellow flesh, resistance to some diseases and is suitable for many uses,

such as table consumption (soup and curry), potato crisps and french fries (Anonymous

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1999; Batt 1994). Another popular cultivar for crisp and chip production is Atlantic,

which is high in carbohydrate and protein but low in sugar and water (Laurence et al.

2000).

The potential for improving yields in the potato industry in Indonesia and indeed, in

most Southeast Asian countries, faces several constraints. The single most important

factor is the lack of good quality seed at an affordable price. Seed is the biggest

expenditure for potato growers. It is estimated that about 36% of the total production

3


costs in the highlands, and more than 50% of total production cost at medium altitude,

goes toward the purchase of good quality seed (Maldonado et al. 1988).

In the tropics, potato cultivars need to meet certain criteria in order to produce high

yields. They must tolerate tropical environmental conditions, such as high temperatures,

short day lengths, short growing periods and a high incidence of pests and diseases.

They must also grow well under low levels of fertilizer and irrigation (Batt 1998).

Bacterial and fungal diseases and insect pests significantly hamper potato production in

the tropics (e.g., Indonesia) and late blight (Phytophthora infestans) is the most

common disease of potato and is the most serious. Potato production can be further

decreased by problems of bacterial wilt (Ralstonias solanacearum), Fusarium dry rot

(Fusarium sp.) and potato virus Y (PVY). Moreover, common pests, such as Potato

Leaf Miner (Liriomyza huidobrensis), Myzus persicae, Macrosiphum euphorbiae, Aphis

sp., Acyrthosihon solani, tuber moth (Phthorimaea opperculela), thrips (Thrips palmi)

and root-knot nematodes (Meloidogyne incognita, M. hapla, M. jawanica and M.

arenaria) and Potato Cyst Nematode (Globdobera sp) also limit the potato yields

(Katayama and Teramoto 1997; Subijanto and Isbagyo 1988). In Indonesia, the use of

poor storage facilities further increases the risk of seed infection and allows for viral

transfer between infected and healthy tubers (Beukema and van der Zaag 1990).

Currently, the potato yields in Indonesia are relatively low, averaging only 13.4 t/ha

(Katayama and Teramoto 1997; Statistics Indonesia 2000). In areas where seed

degeneration rates are high, initially clean seed (0-1% virus), will have 10%, 45% and

100% viral contamination after the 1st, 2nd and 3rd generations respectively. In low

degeneration areas the virus infection rates, in initially clean seed, are initially lower

(2%) but increase to 100% infection after several generations (Beukema and van der

Zaag 1990). The importance of reducing viral infection and transfer is illustrated by the

dramatic decrease in yield of Granola in Batu (East Java). Initially, 30 t/ha are obtained

4


from early generations of clean seed imported from the Netherlands but after the fifth

generation, yield drops to 20 t/ha and is further reduced to 7 t/ha by the 7 th generation

(Batt 1993).

The production of seed potatoes in Indonesia is mainly by an informal seed system

where farmers retain their own seed at harvest. Yields are graded; small tubers for use

as seed and big tubers for sale. Another option is to purchase seed from other potato

growers or from growers that specialize in seed potato production (Batt 1997). This

informal seed system exists nationally and regionally and farmers, well-known as potato

seed growers, supply the demand for seeds. These seed potato farmers are usually

located in the areas with low degeneration rates, such as those at higher altitude (Struik

and Wiersema 1999).

Indonesian growers prefer small whole seed potatoes (Batt 1997). The use of big uncut

tubers for seed is very costly and cutting tuber segments for seed is not practical

because of high humidity, which increases the incidence of diseases entering and

infecting the wounds. The problem is that the highly desired cultivars, such as Atlantic

naturally produce a high proportion of large tubers and tubers very rarely fall into the

small size category (20-55 g). Granola produces more tubers and higher proportion of

small tubers than Atlantic but maximizing yield of small tubers is still required.

Indonesia has attempted to provide better quality seed potatoes by establishing the Seed

Potato Multiplication and Training Project in cooperation with the government of Japan

through the Japanese International Cooperation Agency (JICA) in West Java. The aim is

to reach self-production of affordable virus-free seed potatoes (Anonymous 1999). It is

estimated that this formal seed system will be able to supply one fifth of West Java’s

seed demand (Adiyoga et al. 1999).

5


The annual seed demand for Indonesia can be calculated by multiplying the amount of

cultivated area (hectares), seed rate per hectare and seed renewal rate based on a model

developed by Crissman (1989), TSDj = Aj x Sj x Rj

Where TSDj is the total seed demand for cultivar j (t)

Aj is the area (ha) planted in cultivar j

Sj is the seed rate (t/ha) for cultivar j

Rj is seed renewal rate for cultivar j

Given that the total potato planting area in Indonesia in 2001 was 62,776 ha (Food and

Agriculture Organization of United Nations 2002), the seed rate was 1.5 t/ha and that

the seed renewal rate was 0.20 (on average once every five years), the annual seed

demand is projected to be 18,832 t. However, this simple method is difficult to apply

due to a lack of accurate statistical data on total potato planting area. In Indonesia,

potato production is based on smallholdings where farmers cultivate potato on plots of

less than one-hectare using a multiple cropping system. Seed renewal rates differ from

one area to the next. In the highlands, farmers replace their seeds only once every three

to five years, but in the lowlands the degeneration rates are faster and most farmers use

new seeds for each planting. In this case, the lower effective seed demand is less than

total seed demand, because farmers use their own seeds, and therefore the model has

been modified i.e.

ESDj = TSDj – (TSDj x ISSj) (Crissman 1989)

Where ESDj is the effective seed demand for cultivar j (t)

ISSj is the proportion of seed of cultivar j sourced from the informal seed market.

Based on the new model, seed demand in Indonesia is estimated to be 1,000 t/pa (Batt

1997). In Asia this will approach 31,900 t/pa and is considered to be a realistic

prediction of the actual demand (Batt 1998).

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Currently, importation of seed potatoes into Indonesia is inevitable because there is an

inadequate domestic supply of good quality seed. Granola is imported from Germany

and Holland and Atlantic is imported from Australia and the USA (Adiyoga et al.

1999). The drawbacks of using imported seed are that it is very expensive and only rich

farmers can afford it. Moreover, there are problems associated with imported seed, such

as inappropriate physiological age for planting times in Indonesia related in part to

delayed arrival of seed, especially from Europe (Batt 1997; Schmiediche 1995).

Western Australia is the closest region to Indonesia that grows potato as a main

vegetable crop. Potato growing areas in Western Australia are mainly in the southwest

of the state (Figure 2). The region includes Metropolitan (Gingin to Mandurah),

Mylaup, Donnybrook, Busselton, Margaret River, Manjimup, Pemberton, Mt Barker,

Denmark, Albany, Scott River and Bremer Bay (Burt 1997; Dawson et al.2003).

In 1998-99 the value of potato for the southwest of Western Australia was $30.8

million, more than any other vegetable crop grown in the area (Regional Development

Council of Western Australia 2001).

Wide ranges of cultivars have been grown in Western Australia for different uses. For

the table market, Delaware, Nadine, Désirée, Mondial, Ruby Lou, Spunta and Royal

Blue potatoes are grown. Potato cultivars for processing are Russet Burbank, Ranger

Russett, Kennebec, Shepody, Nooksack and Atlantic (Dawson et al. 2003). Granola is a

relatively new cultivar, currently being tested for the export market. Granola has yellow

flesh and is highly preferable by Indonesian consumers because it is very tasty.

However, Granola has no fresh market in Western Australia.

Western Australia’s fresh domestic potato market is unique. It is regulated by the

Marketing of Potato Act 1946, which ensures the availability of table potatoes all year

round and protects growers and consumers with reasonable returns and price. To do

this, table potato production is characterized by a licensing system according to the area

7


planted, cultivar and time of planting. The Marketing of Potato Act has had a long

history and undergone some revisions. The Potato Marketing Board operated for 35

years (1947 to 1982) in Western Australia and in 1995 was renamed as the Potato

Marketing Corporation under the common name of Western Potatoes (Anonymous

2002).

Western Australia has ideal potato growing conditions that are largely free from many

potato pests and major infectious diseases (e.g., bacterial wilt, potato cyst nematode,

8


late blight and potato virus Y) found in tropical conditions (Schmiediche 1995). Seed

potatoes from Western Australia are available all year and this provides appropriate

physiologically aged seeds that correspond with the planting times in Indonesia. The

strategic geographic location of Western Australia, close to Asian countries like

Indonesia, is also beneficial in terms of transportation costs. These advantages make it

possible for Western Australia to compete with European countries for exporting

cheaper and better quality seed potatoes to Indonesia (Anonymous 2002). Therefore,

importing seed potatoes from Western Australia is an attractive alternative for Asian

countries, such as Indonesia (Batt 1999).

Western Australia first exported 15 tones of seed potatoes to Indonesia in 2000. Seed

potatoes from Western Australia have also been exported to other Asian countries

(Table 1) mainly Mauritius (1,683 t) and Thailand (369 t).

Table 1.1. Export of seed potatoes from Western Australia (t) from 1995- 2001 (ABS 2002).

Yearly export (t) of seed potatoes Country 95/96 96/97 97/98 98/99 99/00 2000/01 Total Brunei 2 2 Hong Kong 3 11 14 Indonesia 15 15 Mauritius 240 431 1,013 1,683Malaysia 90 2 2 10 104 Seychelles 30 30 Thailand 15 67 125 162 369 Total 124 4 290 69 577 1064 2365

The national Seed Potato Certification Standard is a requirement of the potato industry

for domestic and international trade. The Western Australian Seed Potato Scheme was

introduced by the Western Australian Department of Agriculture management through

its Plant Laboratory Business Unit. The purpose of the scheme is to guarantee

9


authenticity of cultivar, disease-free condition of seed and monitor seed quality

(Anonymous 2001).

Both government and private sectors have paid considerable attention to the potential

market of Australian seed potatoes in Southeast Asia. The Department of Primary

Industry and Energy Australia in conjunction with Agriculture Victoria, through the

Toolangi Research Center, has funded a collaborative project to develop the seed potato

industry in both Australia and Indonesia under the Australian-Indonesian Working

Group on Agriculture and Food Cooperation (WGAFC) (Rahman 1996). Performance

of 13 Australian seed potatoes has been evaluated in West and Central Java and the

following recommendations have emerged 1) that the parties, Indonesia and Australia

continue to cooperate to find suitable potato cultivars for Indonesia and 2) that ways to

reduce seed size is a priority task for seed potato growers in Australia (Batt 1997).

In 2002, a project funded by Horticultural Australia Ltd commenced between the

Department of Agriculture Western Australia and Indofood-Fritolay aimed at improving

potato production (small potatoes). Several key problems were identified including a

lack of good quality seed, poor storage facilities and a high incidence of disease.

Furthermore, the development of methods aimed at improving potato crop management

was also proposed. Reciprocals visits have since occurred, which examined potato

production in both countries. Growers, researchers and government officials have been

involved in these studies.

The primary objective of this project was to develop techniques aimed at increasing the

yield of small round seed potatoes, (Solanum tuberosum L.). Specifically, it sought to

increase the yield of seed tubers weighing 20-55 g in the cultivars Atlantic and Granola

without reducing total tuber yield.

10


Australian Bureau of Statistics (2002)’ Agriculture Australia 2000-2001.ABS Catalogue

7113.0.(ABS:Canberra)

Dawson PD, McPharlin IR and Howes M(2003) Table and seed potatoes from Western

Australia, at a glance. Bulletin 4586 (Western Australian Department of Agriculture).

11

Chapter 2

Literature Review

2.1. Potato morphology and anatomy

Potato (Solanum tuberosum L.) is a member of the Solanaceae along with many other

crops including tomato, tobacco, pepper and eggplant. Potato plants (Figure 2.1) are

herbaceous dicotyledons consisting of stems, leaves, tubers, stolons and roots (Jadhav

and Kadam 1998). Mature leaves are compound, each with several leaflets (Dean 1994).

The number of leaflets varies with cultivar but there are usually 3 or 4 pairs of large

leaflets and one terminal leaflet (Kadam et al. 1991).

Figure 2.1. The morphology of potato plant. Modified from Kadam et al. (1991).

Chapter 2 Literature review

Potato tubers (Figure 2.2) are modified underground stems, which act as storage organs

(Beukema and van der Zaag 1990). Each tuber is attached to a stolon at the heel end and

the original stolon apex is referred to as the rose end. The skin of the tuber is designed

to protect but does contain many lenticels for respiration (Beukema and van der Zaag

1990). On the surface of the tuber there are also buds, arranged in groupings that are

commonly referred to as a potato eye. At the rose end of the tuber apical buds are

formed from which new shoots may arise. Usually, each eye contains one main lateral

bud, which is the biggest, and one smaller bud on each side of the main lateral bud and

buds grow into sprouts (Cutter 1992). Potato tubers have many eyes in their surface and

these are arranged in a phyllotactic spiral (Allen et al. 1992). The numbers of eyes that

develop on each tuber depend on tuber size and cultivar; large tubers generally have

more eyes than small tubers (Struik and Wiersema 1999).

In longitudinal section, the tissues of the tuber include epidermis, periderm, cortex,

vascular ring, perimedullary region and pith (Figure 2.3). The periderm may be 5-15

cell layers thick (Beukema and van der Zaag 1990) and the perimedullary and pith

region contain a large amount of starch (Kadam et al. 1991). Potato plants generally

have a fibrous root system and when grown from true seed the root system is composed

of a slender taproot with lateral branches. Potato plants are more commonly grown from

seed tubers and have groupings of three adventitious roots that arise from nodes of

underground stems (Kadam et al. 1991). During early plant growth the roots are

confined to the surface soil layers and later turn downward reaching 1.5 m in depth

(Kadam et al. 1991; Cutter 1992).

12


Young sprouts develop into mature stems, which are the aboveground and belowground

axis of plant. Above ground stems are sites of petiole growth and contain leaves. Stems

and leaves together are called haulm. Stems can be unbranched or branched (Burton

1989) and are triangular or quadrangular in cross section (Dean 1994). The main stems

develop directly from seed tubers, and more than one main stem often arises

13


from each eye. Branch stems can grow from main stems either above or below ground

(Struik et al. 1990). Stem characteristics, such as number and thickness are of practical

significance because potato plants with only a few, thick stems, usually produce a few

large tubers. In contrast, potato cultivars with several, thin stems, usually produce many

small tubers (Pushkarnath 1976).

The stolon is an underground lateral shoot with reduced leaf growth and a hook tip,

which grows diageotropically (Vreugdenhil and Struik 1989). Apical and sub apical

regions are located proximal and distal from the stolon hook (Figure 2.4). There are 3

types of stolon, i) primary stolons, that grow from node of main stems, ii) lateral

stolons, that grow from lateral stems and iii) branch stolons that arise from the primary

and lateral stolons (Wurr et al. 1997).

Figure 2.4. A stolon develops a hook showing apical and sub apical regions on either side of the hook (Viola et al. 2001). 2.2. Seed Potatoes

Potatoes are vegetatively propagated either as segments or as whole tubers (Hide 1986).

Stem cutting are also used as a means for rapid propagation (Struik and Wiersema,

1999). Another means of potato propagation is by true seed but high genetic variation

and low yield limit the use of true seed (Dean 1994). In the USA, Spain and Australia

14


large seed-tubers are cut (Allen et al. 1992). Cutting is beneficial because it reduces

seed cost and improves multiplication rates (Beukema and van der Zaag 1990).

However, in most European (Allen et al. 1992) and Southeast Asian countries

(including Indonesia) seed decay is a problem and potato propagation is by whole seeds

(Batt 1997) to avoid seed decay. In these countries methods for maximizing the yield of

small seed tubers are important since the desirable cultivars naturally produce large

tubers.

Seed size can be expressed as tuber length (mm) or weight (g). The optimum size for

planting as whole seed ranges from 20 to 60 mm (Allen et al. 1992). In Indonesia a

smaller seed size is more desirable and ranges from 20 to 55 g.

Atlantic and Granola are widely cultivated in Indonesia. The cultivar Atlantic is an

American potato cultivar with a round tuber, white-netted skin and white flesh (Figure

2.5). It has high yield and moderate dormancy. It has tolerance to powdery scab

(Spongospora subterranea) and potato cyst nematode and moderate resistance to

common scab (Streptomyces scabies) but it is susceptible to hollow heart, black spot

bruising and internal brown spot (Gratte and Paust 1990). It is grown for processing

with good chip colour, a high specific gravity and in Australia is a major crisping

cultivar. It naturally produces only a few, large tubers (Webb et al. 1978) and in the

Southern Australian climate produces approximately 5 tubers per plant (SARDI 2001).

This cultivar is very suitable for growth in the tropics, where it has a high canopy

photosynthetic rate and high yield (Bhagsari et al. 1988). Therefore, this cultivar is well

suited for growth in the tropics of Indonesia.

Granola is a European cultivar originating from Germany. Granola tubers are round to

oval with yellow-netted skin and yellow flesh (Figure 2.5). It is a medium maturing

cultivar and grows well in a wide range of climates, including the tropics. Granola has

resistance to blackleg, early blight, common scab, bruising and hollow heart. It

15


produces high yield and a large number of tubers per plant with long dormancy (Potato

Working Group 2001). Granola grows well in Indonesia but the highest yield (30 t/ha)

was achieved using disease free seeds. Generally however, yields are much lower than

this (approximately 13.4 t/ha) due to the use of degenerated seed, which have

accumulated diseases (Batt 1997).

Figure 2.5. Atlantic tubers are round with white flesh. Granola tubers are round to oval with yellow flesh.

2.3. Stolonization

Stolons are the sites of tuber initiation, therefore their growth and development is very

important. Stolons emerge from parts of underground stems. The first stolons develop at

the basal node followed by stolon development at upper nodes (Wurr et al. 1997). Both

cell division and expansion contribute to stolon elongation. In the apical region of

stolons, cells divide in the transverse plane, thus generating stolon elongation (Duncan

and Ewing 1984; Xu et al. 1998a).

Influence of hormones on stolonization

The main hormone that promotes stolon growth is gibberellic acid (Koda and Okazawa

1983a; Xu et al. 1998b). The endogenous GA levels are high in elongating stolon tips

(Koda and Okazawa 1983a; Xu et al. 1998b) and specifically, GA1 is the active GA for

16


stolonization (Xu et al. 1998b). In many plant species (Metraux 1987; Jacobsen et al.

1995) including potato, GA is an important hormone for regulating cell elongation

(Hammes and Nel 1975). Applied GA increases stolon length (up to 268 cm/plant)

(Bodlaender and van de Waart 1989; Sharma et al. 1998b), dry weight (Hammes and

Nel 1975) and induces stolon branching (Bodlaender and van de Waart 1989). Altering

stolon numbers by application of GA is potentially a very important tool for increasing

tuber numbers. However, in field-grown potatoes, stolon number is very hard to

measure because stolons are damaged when potatoes are dug. Stolon branching is also

important but again it is difficult to quantify in field-grown potatoes. Moreover, in the

large field trials potatoes are usually machine harvested which destroys stolons.

Growth of stolons is inhibited by anti-gibberellins. Paclobutrazol is an anti-gibberellin

frequently used in potato tissue culture (Simko 1991, 1993, 1994) and pot culture

(Balamani and Poovaiah 1985; Bandara and Tanino 1995; Bandara et al. 1998).

Paclobutrazol inhibits stolon growth reducing stolon fresh and dry weight. Other anti-

gibberellins, such as 2-chloroethyltrimethyl ammonium chloride (CCC) also alter stolon

growth in pot-grown (Abdala et al. 1995) and in field-grown potatoes (Sharma et al.

1998b).

Influence of photoperiod and temperature on stolonization

Photoperiod influences stolon elongation. A long photoperiod promotes elongation,

whilst a short photoperiod inhibits stolon elongation (Vreugdenhil and Struik 1989).

This is mediated by changes in the endogenous levels of GA in stolons. The GA

concentration decreases under short photoperiod and it increases under long photoperiod

(Machackova et al. 1998). Temperature also influences stolon growth. High soil

temperatures in the field delay stolon development but increase stolon numbers and

yield (Midmore 1984). Furthermore, high temperatures in controlled environments

stimulated stolon branching (Struik et al. 1989a).

17


2.4. Tuber initiation

When stolon elongation ceases, tuber growth is initiated by a distinct swelling of the

sub-apical region of stolon tips (Koda and Okazawa 1983a; Xu et al. 1998a; Jacson

1999). The morphological and anatomical changes that take place in the stolon during

tuberization have been classified into four stages (Figure 2.6) (Koda and Okazawa

1983a; Xu et al. 1998a).

Stage A has elongating stolon tip without signs of swelling. The elongation of the apical

region of the stolon is due to cell division in the transverse plane and cell elongation.

Cross-sectional cell number along the stolon (length) diameter is similar and no

longitudinal cell division has occurred (Xu et al. 1998a). During stage B, swelling of the

sub-apical region commences and the stolon tip has not straightened out yet. Cell

number between swelling and non-swelling parts of the stolon are similar but the cell

width at the swollen region is 60% greater. This indicates that cells of the swollen tip

have expanded radially (Koda and Okazawa 1983a; Xu et al. 1998a). At this stage

tubers are about twice the stolon diameter (Firman et al. 1991) and in this thesis this is

referred to as early tuber initiation. In Stage C stolons are fully swelled. Swelling of the

sub-apical region continues mainly due to cell division. This stage is also marked by

straightening out of the hook (Koda and Okazawa 1983a). Cell number increases

substantially in the three main regions of the tuber (i.e. the cortex, perimedulary region

and pith) (Xu et al. 1998a; Cutter 1992). In this thesis, stage C refers to late tuber

initiation, where the longest tuber is 10 mm. Stage D is where tubers are approximately

20 mm in diameter (Xu et al. 1998a). Tuber enlargement is caused by cell division and

cell enlargement (Koda and Okazawa 1983a; Xu et al. 1998a). Longitudinal cell

division stops whilst cell enlargement continues until tubers reach marketable size

(Struik et al. 1988; Xu et al. 1998a).

18


Figure 2.6. Stolon development. Stage A (elongating stolon tips), stage B (swelling stolon tips), stage C (fully swollen stolons) and stage D (young tuber 1-2 cm across) (Koda and Okazawa 1983a).

All processes mentioned above, including stolon initiation, growth, ceasation of growth,

tuber initiation and tuber bulking are referred to as tuberization (Vreugdenhil and Struik

1989). In addition to the morphological and anatomical changes occurring during

tuberization (Xu et al. 1998a) there are also changes in endogenous hormone levels

(Koda and Okazawa 1983a) (described in sections 2.8.5.1 to 2.8.5.6). Starch deposition

takes place from Stage A to D with the most rapid deposition occurring from stage A to

C. During stage D deposition decreases (Ross et al. 1994).

2.5. Dormancy

Potatoes are harvested when they reach marketable size and then they enter a phase of

dormancy (Burton 1989). The dormant period is finished when tubers have developed at

least one sprout (2-3 mm) (van Ittersum and Scholte 1992b). Although dormancy starts

at tuber initiation (Claassens and Vreugdenhil 2000) the more practical approach is to

19


use the time after harvest during which sprouts cannot grow as the date marking the

onset of dormancy (Cho et al. 1983b). In this thesis dormancy is referred to in this

manner.

When a tuber is dormant sprouts cannot grow, even if tubers are placed under

conditions that promote sprouting (i.e. darkness, 15-20oC and 90% relative humidity)

(Reust 1986). This type of dormancy is called innate, absolute or true dormancy and

also endodormancy. It is characterized by suspension of growth by internal factors of

the dormant structure (Lang 1987; Suttle 1998; Struik and Wiersema 1999). A second

type of tuber dormancy is called paradormancy, which is suspension of growth due to

unfavorable factors within the plant but outside the dormant structure. A third, type of

dormancy is ecodormancy, which is caused by unfavorable environmental conditions

such as low temperatures (4oC) that prevent sprouting (Lang 1987).

The length of endodormancy period is greatly influenced by temperatures and is also

cultivar dependent. Generally the period of endodormancy is longer when tubers are

stored at lower, rather than higher temperatures (van Ittersum and Scholte 1992b).

Temperatures during the growing season also influence the subsequent length of

endodormancy. Endodormancy is shorter when hot, dry growing conditions prevail

(Krijthe 1962). Tubers grown under high temperatures of spring have shorter dormancy

than tubers grown under cooler temperatures of autumn (Burton 1963). As with so many

other properties of tubers the influence of growing temperature is cultivar dependent

(van Ittersum and Scholte 1992a).

Influence of hormones on dormancy

As discussed for stolonization, tuber dormancy in potatoes is also promoted and

inhibited by hormones. During tuber initiation, until tuber growth is completed, the

endogenous GA levels are low (Koda and Okazawa 1983a; Xu et al. 1998b) and remain

20


low during tuber endodormancy (Smith and Rappaport 1961). The GA levels increase

gradually as dormancy progresses and a high GA concentration is related to bud break

(de Bottini et al. 1982). Application of GA breaks endodormancy and releases bud

growth (Claassens and Vreugdenhil 2000; Fernie and Willmitzer 2001).

Cytokinins are growth promoters, which can terminate endodormancy in potato tubers

(Turnbull and Hanke 1985a; Sonnewald 2001). There are eight endogenous cytokinins

in endodormant potato buds, namely zeatin riboside-5’-monophosphate (ZRMP), zeatin-

o-glucoside (ZOG), zeatin (Z), zeatin riboside (ZR), isopentenyl adenosine-5’-

monophosphate (IPMP), isopentenyl adenine-9-glucoside (IP-9-G), isopentenyl adenine

(IP) and isopentenyl adenosin (IPA) (Suttle 1998). The termination of endodormancy

in potato tubers by storing them at 20oC is accompanied by a considerable increase in

the concentration of Z, ZR, IPMP and IP-9-G (Suttle 1998). Increased cytokinin levels

in buds, which leads to the commencement of sprouting, is mediated through cell

division (Turnbull and Hanke 1985b; Sukhova et al. 1993). Dose response of applied

cytokinin isomers (cis-zeatin and cis-zeatin riboside) to sprouting in dormant potatoes

indicates that sensitivity increases with prolonged time of post-harvest storage. Dormant

tubers are insensitive to the applied cytokinin isomers immediately after harvest.

Sensitivity increases from 53 days onward in cool storage at 3oC, as indicated by

increased sprouting. Once endodormancy is completely broken the sensitivity decreases

(Suttle and Banowetz 2000).

Endogenous abscisic acid (ABA) is an inhibitor of bud break from dormancy. The heel

end of dormant potato tubers is the major source of ABA (Ji and Wang 1988). During

post harvest storage at 4oC, the endogenous ABA content changes in buds of dormant

potato tubers. From the beginning to about 20 days in cool storage endogenous ABA

content increases sharply and reaches a peak after 60 days, thereafter decreasing when

endodormancy is broken and sprouting commences (Cvikrova et al. 1994). These

21


changes are also observed in whole dormant tubers during post-harvest storage.

However, ABA concentration in eyes of dormant potato tubers increases at the end of

dormancy and this is accompanied by sprouting of potato tubers. This suggests that

ABA is not the sole factor maintaining dormancy (Sorce et al. 1996).

Ethylene prolongs dormancy. Potato tubers produce a small amount of ethylene during

post harvest storage and this extends dormancy (Poapst et al. 1968; Rylski et al. 1974).

Application of ethylene-releasing compounds, such as 2-chloroethylphosphonic acids

prolongs dormancy in potato (Cvikrova et al. 1994). Auxin reduces endodormancy in

potato tubers. The concentration of free indole acetic acid (IAA) in potato eyes

increases toward the end of the dormancy period and implies that endogenous IAA

promotes dormancy breaking (Sorce et al. 2000). These more recent findings contrast

with earlier studies that suggested endogenous IAA was not involved in reducing

endodormancy in potatoes (Sukhova et al. 1993).

2.6. Apical dominance

In general terms apical dominance refers to i) the control of lateral bud growth by the

apex, ii) dominance of one growing shoot over another and iii) the influence of the apex

on the orientation of branches and leaves (Martin 1987). As already discussed, in potato,

each tuber has several eyes and each eye usually has three buds. Thus in potato, apical

dominance refers to suppression of lateral sprout growth between eyes by a single or

multiple apical sprouts on a single tuber. Apical dominance in potato can also refer to

the suppression of lateral sprout growth within an eye by the largest sprout in the eye.

The extent of the suppression of other sprouts depends on the degree of apical

dominance (Kumar and Knowles 1993).

22


Once tuber dormancy has been broken they are in an apically dominant state,

characterized by single-sprout growth (Mikitzel and Knowles 1990; Kumar and

Knowles 1993). There is very little or no lateral bud growth when the apical bud is

intact but when the apical bud is damaged or removed the lateral buds grow out (Cline

1994). Apical buds are sources of inhibition for lateral bud growth. Apical portions of

plants or growing shoot apices are sites of auxin synthesis and inhibit the growth of

lateral shoots (White and Mansfield 1977; Hilman 1984). There are conflicting theories

about the roles of auxin in apical dominance (Cline 1994). One theory suggests that

auxin produced by the growing shoot apex is transported downward and enters lateral

buds directly inhibiting their growth. Alternatively, other authors suggest that auxin

synthesis increases the sink strength of the shoot apex and thus there are insufficient

nutrients for lateral buds to grow. Another theory proposes that auxin synthesis induces

secondary products and it is these secondary products that enter lateral buds and inhibit

their growth.

2.6.1. Manipulation of apical dominance

One way to increase tuber number in potato is by increasing stem number (Struik et al.

1990). Since apical dominance generally limits stem number in potato, treatments which

reduce apical dominance and increase stem number are potentially important for the

cultivation of small seed potatoes. Physical treatments, such as desprouting (Holmes

and Gray 1971) or apical shoot removal (Hay and Hampson 1991; Harrington 2000),

nicking or pinching and decapitating (McKeown 1994) are means of increasing sprout

and stem number. Principally, these treatments damage shoot apices leading to reduced

auxin synthesis resulting in outgrowth of lateral buds and thus more stems. Although

these manual treatments can increase sprout, stem and tuber number they are less

practical for large scale application. Therefore, investigation of the influence of storage

23


duration and chemical application to reduce apical dominance in potatoes could be very

useful for seed potato production.

Chemical sprout suppressants, such as carvone (L-carvone) which induces the growth of

lateral buds and branching of main sprouts, may reduce apical dominance (Oosterhaven

et al. 1995). The mode of carvone action in reducing apical dominance is by damaging

larger primordial sprouts and this allows lateral sprout growth (Baker et al. 2002). With

certain cultivars the efficacy of carvone for stimulating sprout and stem growth is as

affective as manual desprouting (Hartmans and Oosterhaven 1998). This chemical has

never been tested with the cultivars Atlantic and Granola.

Plant growth regulators are also used to reduce apical dominance in plants. Amongst the

various growth regulators, gibberellic acid (GA) is the most commonly used in potato

(Holmes et al. 1970; Claassens and Vreugdenhil 2000). Application of GA results in

multiple sprout growth, which indicates a loss of apical dominance. The mode of GA

action in breaking apical dominance is probably via interactions with other hormones.

For example, if GA3 is applied to decapitate stumps of pea (Pisum sativum) the growth

of lateral buds is stimulated. However, when GA3 and IAA are applied together, to

decapitated pea stumps, the growth of lateral buds is inhibited, probably because GA3

increases the release of IAA from the dominant apex (Jacobs and Case 1965).

2.7. Physiological age

Aging in potato tubers means maturation with time (Hartmans and Van Loon 1987).

Chronological age of tubers is accumulated from time of harvest, which can be

quantified by units such as days, weeks or months (Kawakami 1980). The physiological

age of tubers is the physiological state of tubers at any given time from initial dormancy

until the incubation period. It is influenced by storage conditions, environmental factors

during growth and chronological age (Kawakami 1962). It is important for seed

24


potatoes to have proper aging before planting to ensure maximum yield. This is usually

4-6 months of aging but the appropriate aging period is cultivar dependent (van der

Zaag and van Loon 1987). In cases where less than four months aging has been allowed,

juvenile degeneration occurs, characterized by very low sprouting. Moreover, excess

aging (more than 6 months) leads to senile degeneration and is associated with

decreased productivity (Kawakami 1962). Therefore the appropriate aging in Atlantic

and Granola required for maximum sprout and stem number is needed.

A desired physiological age can be created by altering storage temperature and or

storage duration (Mikitzel 1990). Seeds age with prolonged storage and age faster at

high, rather than low temperatures (Bodlaender and Marinus 1987; van Loon 1987; van

der Zaag and van Loon 1987; Knowles and Botar 1992a). Physiological age influences

sprouting characteristics of seed tubers. Sprout number can be increased by extending

storage duration but prolonged storage reduces it. Again, duration of storage for

maximum sprout number depends on the cultivar. Generally, rapid aging cultivars, such

as Jaerla need shorter storage duration (300-500 days at 4oC) whilst slow aging

cultivars, such as Désirée need longer storage times (400-460 days at 4oC) (Hartmans

and Van Loon 1987). Generally, tubers of the same cultivar, when stored at 4oC,

produce more sprouts than those stored at 12oC. Under glasshouse conditions the

maximum number of sprouts in Désirée tubers (6 stems per plant) was achieved after

about 410 days storage at 4oC. Storage at 12oC leads to a maximum of 3 stems per plant

after 200 days storage (Bodlaender and Marinus 1987). The results obtained under field

conditions are similar, where maximum stem numbers (5.8) per plant were produced

after 240 days storage at 4oC, whilst fewer stems (3.2) per plant were produced after the

same storage period at 12oC (van Loon 1987).

Prolonged storage at a constant temperature of 4oC induces ageing in potatoes. Changes

in seed sprouting are observed in tubers of different ages. For example, Russet Burbank

25


seed tubers are endodormant for 3 months after harvest. When dormancy has just

broken, a single sprout emerges from the apical eye (apical sprout) and this sprout

inhibits the growth of other apical sprouts within the same eye and other apical sprouts

in other eyes. This apical dominance lasts for about 5 months in cool storage. By

increasing the storage duration (6-9 months) more lateral sprouts grow (i.e. up to one

sprout from each eye) which indicates a loss of apical dominance. After 15 to 21 months

storage, multiple sprouts grow from each eye indicating a further loss of apical

dominance. Extending storage period beyond 28 months results in formation of tiny

potatoes, directly on the seed tubers (Kumar and Knowles 1993).

Seed age influences the rate of stem (sprout) emergence. Under controlled conditions

the stems of seeds stored at 4oC emerge faster than those from seed stored at 12oC and

this varies across cultivars. For example, after 100 days storage at 4oC, the seed of

Désirée had produced half the final number of stems 16 days after planting (DAP). In

contrast, when seed is stored for the same period at 12oC, a similar proportion of stems

is not developed until 24 DAP (Bodlaender and Marinus 1987). Under field conditions

the stems emerge quicker when seed is stored at 4oC compared with than when seed was

stored at 12oC (van Loon 1987).

The total number of stems developed on potato seed tubers is also influenced by their

physiological age, both under controlled conditions and in field–grown plants. Old seed

produces more stems than young seed (Kawakami 1980; Kumar and Knowles 1993).

There is a linear relationship between stem number and seed age and in field-grown

potatoes, the older the seed, the more stems are produced (Knowles and Bottar 1991;

Knowles and Botar 1992b).

The increase in tuber yield with tuber age is mediated through earlier sprout emergence

and earlier tuber initiation. Both of these provide for a longer period for tuber bulking

(Asiedu et al. 2003). This is particularly relevant where the growing period is short

26


(Knowles and Botar 1992b). Aging of seed reduces apical dominance, increases the

number of sprouts and stems and in turn increases the number of tubers (Iritani et al.

1983; Knowles and Bottar 1991). Auxin synthesis and higher concentrations of auxin in

apical buds are associated with a stronger apical dominance. Older seed potatoes have

higher IAA oxidase activity than younger seed, so they have a greater ability to

catabolise IAA and thereby have reduced endogenous auxin levels (Kumar and

Knowles 1993).

Tissue age also influences the polar transport of auxin in plants (Goldsmith 1977;

Jacobs 1979; Suttle 1988). Generally, the ability of tissue to transport endogenous or

exogenous auxin basipetally, decreases with increasing age of tissues. For example, the

recovery of radioactivity labeled auxin in petioles of coleus (Veen and Jacobs 1969)

cotton (Devenport et al. 1980) and sunflower (Suttle 1991) decreases with increasing

age. In potato, the capacity of auxin to be transported from apical to lateral sprouts of

seed tubers also reduces with increasing chronological age of the tuber (Kumar and

Knowles 1993). Thus, high auxin catabolism, coupled with decreased capacity for

basipetal auxin transport, in aged seed potatoes, reduces auxin availability and

translocation to lateral buds. This would in turn release the lateral buds from correlative

inhibition by the apical buds (Kumar and Knowles 1993).

Seed age influences the mobilization of carbohydrate reserves. Although old seeds (19

months at 4oC) produce more sprouts (stems) than young seeds (7 months at 4oC) the

dry matter content of old seeds on a shoot basis is lower. The rate of starch degradation

is not influenced by tuber age, but old seed accumulates 2.3 fold more reducing sugars

(glucose and fructose) than young seed (Mikitzel and Knowles 1989a). This implies that

either, the synthesis of sucrose from glucose and fructose is less efficient in old

compared with young seeds, or that sucrose hydrolysis to glucose and fructose is faster

in old seed. The high competition between shoots for limited carbohydrate supplies

27


results in lower dry weights of individual shoots. Overall, the sink strength of shoots

derived from old seed is lower than that of young seed (Mikitzel and Knowles 1989a).

The low carbohydrate mobilization rates are probably related to a low polyamine

content in old seed potatoes (Mikitzel and Knowles 1989b). Polyamine is known to

regulate plant growth and a high concentration is correlated with actively growing buds

(Kaur-Sawhney et al. 1982).

There is also an influence of meristem age on several physiological factors related to

mineral nutrition. For example, meristems of potato from old seed are less efficient in

accumulating reduced nitrate-N and free amino acid-N than young seed (Knowles

1986). Moreover shoots developed from old seeds have low sink strength for seed

nitrogen. The less efficient use of accumulated nitrate-N and free amino acid-N by old

seeds, together with competition between shoots for nitrogen, results in reduced shoot

vigor (Knowles 1987).

Seed storage at 4oC is common practice in Western Australia in order to create an

appropriate physiological age of seeds before planting. As aging of tubers progresses

during storage, the apical dominance of seeds reduces and this allows the outgrowth of

lateral buds, which in turn increases sprout and stem growth. Given the importance of

Atlantic and Granola cultivars to Indonesia and the value of their seed export from

Australia, there is very little information about appropriate storage duration of seed

tubers at 4oC to produce the highest number of sprouts and stems (Struik et al. 1990).

2.8. Factors influencing potato tuberization

Tuberization in potato plants is a complex process, which is influenced by photoperiod,

temperature, water availability, and soil texture and plant growth regulators. These

factors can act alone but tend to interact with to each other to regulate tuberization.

28


2.8.1. Photoperiod

There is high variation in the response of tuber initiation of potato genotypes to

photoperiod (O'Brien et al. 1998). The influence of photoperiod on potato tuber

initiation under field conditions is difficult to study because variation in quantity and

quality of radiation changes during the day and with photoperiod. As a result, the

influences of photoperiod on potato tuberization have been mostly studied under

controlled conditions, particularly using potato cuttings (Ewing 1978; Synder and

Ewing 1989).

Generally, potato plants exposed to short photoperiods have short stolons, form tubers

early and have small stems and leaves. In contrast, plants exposed to long photoperiods

have longer stolons, form tubers later and have large stems and leaves with a greater

number of branched shoots. Furthermore, the time of flowering is delayed when potato

plants are exposed to longer photoperiods (Beukema and van der Zaag 1990;

Machackova et al. 1998). In terms of what constitutes short and long photoperiods in

potato; short photoperiods are 10 to 13 h whilst long photoperiods are greater than 14 h

(Haverkort 1990).

Photoperiod influences tuberization in part, through changes in the levels of endogenous

hormones. Potato plants exposed to short days have very low levels of endogenous

gibberellins in leaves, stems, roots, solons and tubers (Machackova et al. 1998). In fact,

the initiation of tubers is associated with declining levels of endogenous gibberellins

(Koda and Okazawa 1983a; Xu et al. 1998b)

Photoperiod also influences the abscisic acid (ABA) levels in potato plants. When

photoperiods are short the endogenous levels of ABA are high in all organs (i.e. leaves,

stems, stolons, roots and tubers). High levels of ABA, and a particularly high GA:ABA

ratio promote tuberization. When photoperiods are long the ABA levels are lower (i.e.

20% of the levels during short photoperiods) (Machackova et al. 1998). The level of

29


ABA-like substance (an extracted fraction with chromatographic properties similar to

that of ABA) in swelling stolons is high compared with non-swelling stolons suggesting

that high ABA is related to tuberization (Koda and Okazawa 1983a).

As indicated earlier, potato genotypes differ in respect to photoperiod sensitivity. Some

genotypes have critical photoperiods that are relatively long compared with other

genotypes. When the photoperiod is longer than the critical-length, tuberization will be

inhibited and the growth of foliage is promoted. If the photoperiod length is less than

the critical-length, tuber growth will be promoted and foliage growth is inhibited. The

critical photoperiod for some European potato cultivars have been measured and are

between 15 and 17 h for cvs. Eerlasting, Bintje, Gineke and Eigenheimer and between

13 and 14 h in Alpha (Beukema and van der Zaag 1990).

The photoperiod influences the accumulation of starch in potato leaves. Potatoes leaves

exposed to short days (8 h light) accumulated much more starch compared with that

when exposed to long days (18 h light). The export of assimilates from leaves to tubers

is also greater under short days compared with that at long days (Lorenzen and Ewing

1992). The efficiency for converting photosynthetically active radiation into dry matter

is higher under short days compared with that under long days. Consequently, the tubers

formed during short days have a greater dry mass than those developed under long days

(Lorenzen and Ewing 1990).

2.8.2. Temperature

Potato is a cool-climate crop (Haverkort 1990) so it requires relatively low temperatures

to achieve maximum net photosynthetic rates but this varies amongst cultivars (Ewing

1981; Prange et al. 1990). The optimum temperatures for net photosynthesis in potato

ranges from 16 to 25oC (Ku and Edward 1976). Cultivars of European potato require

temperatures around 20oC in order to achieve optimum photosynthetic rates. When

30


temperatures rise to 25oC the photosynthetic rates are reduced by approximately 25%

and at 30oC the respiration rate doubles (Burton 1981).

Temperature is also important for the process of tuberization. Low temperatures

promote tuberization by increasing the number of stolons developed underground, thus

increasing the sites for tuber initiation. The greatest number of stolons and tubers

generally develop at constant temperature of 15oC or 15/20oC (night/day) (Borah and

Milthorpe 1962).

At both short and long photoperiods, high temperatures inhibit tuberization by changing

photosynthate partitioning. Under high temperatures, 14C partitioning studies show that

less photosynthate goes to tubers and more goes to leaves, stems and roots (Jacson

1999) resulting in lower tuber yield (Ewing 1981). Even higher temperatures (32oC)

combined with a long photoperiod (16 h) further decreases 14C partitioning to tubers

(Wolf et al. 1990). Under field conditions, dry matter partitioning to tubers increases as

temperature decreases and vice versa (Manrique and Bartholomew 1991).

Although low temperatures are recommended for potato tuberization, potatoes can be

grown in tropical regions where temperatures are relatively high. This is because the

day length in tropical areas is short, approximately 12 h (Haverkort 1990). Short days

favour early tuberization and compensate for the influence of higher temperature on

growth in tropical regions (Beukema and van der Zaag 1990). However, the yield in hot,

tropical regions is generally lower because high soil temperatures increase the incidence

of diseases responsible for seed rot (Midmore 1992). Therefore, in the tropics, such as

Indonesia, the use of whole seeds is required in order to minimize the incidence of

disease.

When potato plants are grown at very high temperatures or sprayed with GA at high

concentration, they fail to tuberize. In contrast, the application of growth inhibitors,

such as ABA and growth retardants such as CCC, promote tuberization both at low and

31


high temperatures. It seems that the influence of temperature on tuberization is by

regulating the levels of endogenous hormones (Menzel 1980). High temperatures inhibit

tuberization probably by increasing endogenous gibberellins, which are synthesized in

shoot apices (Menzel 1981). Decapitation of buds from potato plants grown at high

temperature (32oC) results in outgrowth of axillary buds, but inhibits tuberization. The

application of chemical pruning agents, such as 2,3,5-tri-iodobenzoic acid maleic

hydrazide and 1-decanol also suppresse the growth of axillary buds and promote

tuberization (Menzel 1981). Other treatments to reduce foliage growth need to be

explored, such as the use of herbicides. Herbicides, such as paraquat + diquat applied at

high concentrations can desiccate foliage (Summers 1980; Ashton and Monaco 1991).

At much lower concentrations they may kill young shoots and thus may reduce GA

synthesis. A reduction in endogenous GA at the appropriate time could promote

tuberization.

Soil temperatures influence plant growth as well as tuberization. In the hot lowland

tropics the maximum temperatures range from 27 to 31oC. Soil temperatures can be

modified by using reflectants to reduce temperatures and polyethylene sheets to increase

temperatures. Cooling soils accelerates sprout emergence, increases tuber initiation and

increases final yield. Conversely, heating soils inhibits emergence, tuberization and

reduces final yield. Genotype selection and environmental manipulations are both

important aspects to maximize yield (Midmore 1988b). Cultivars must be able to cope

with high temperatures during emergence (Midmore 1992). In the hot tropics, mulching

with rice straw (Midmore et al. 1986a; Midmore et al. 1986b) and shading (Midmore

1988b) can also help to reduce soil temperature and water loss thus increasing yield.

32


2.8.3. Irrigation

Optimal irrigation is necessary for good potato yield (Evans and Neild 1981) and hence

high economic returns (Waterer 1997). Water is essential for plant physiological

processes, such as transpiration, photosynthesis, cell enlargement and enzymatic

activities. Water deficit can inhibit or completely halt some of these processes and

depending on the severity, may reduce yield (van Loon 1981). Even short periods of

water deficit can reduce yield (Miller and Martin 1987). Under water deficit, stomata

tend to close (Epstein and Grant 1973) leading to a reduction in transpiration and

photosynthetic rates (Costa et al. 1997), which reduce dry matter production and tuber,

yield (Levy 1983; Jefferies and Mackerron 1987a; MacKerron and Jefferies 1988;

Burton 1989).

Water deficit also reduces the number of tubers developed, especially if water stress

occurs during tuber initiation. The influence of water stress on the number of tubers

developed is also cultivar dependent (MacKerron and Jefferies 1986; Jefferies and

Mackerron 1987a). Cultivars that tend to produce a relatively large number of tubers

(e.g., 10-12 per plant), such as Alpha, Elvira and Désirée when adequate water is

supplied for growth, reduce the number of tubers (6-8) developed when water is limiting

(Levy 1983). This is undesirable for seed potato production because the aim is to

promote the development of large numbers of relatively small tubers.

In addition to the number of tubers developed, the quality of the tubers is also

influenced by water management. Careful water management at the time of tuber

initiation not only results in the maximum marketable yield but also helps control

common scab disease. The incidence of common scab increases with decreasing water

input during tuber initiation (Wilson et al. 2001). Malformation of tubers results from

water deficit and tubers become pointed at the heel end. These tubers generally have

low starch content and a high reducing sugar content which influences their fry quality

33


(Iritani 1981). Water deficit also causes external defects, such as knobs and growth

cracks and internal defects, such as hollow heart (Martin et al. 1992). Atlantic is

susceptible to hollow heart (Gratte and Paust 1990) therefore appropriate irrigation

scheduling for this cultivar is essential.

If insufficient water is applied, external and internal defects may occur thus reducing

yield. Therefore, potato cultivation under the field conditions in Western Australia has a

recommended watering schedule using tensiometers (Hegney and Hoffman, 1991).

Shortly after shoot emergence tensiometers are installed at 30 cm depth and other

tensiometers at different depths depending on rooting depths and soil types. Potatoes are

irrigated when tensiometer readings fall to 20 cb (trigger point) at 30 cm depth. The

amount of water applied is 20 mm to recharge the crop zone (Hegney and Hoffman,

1991).

2.8.4. Soil properties

Soil texture indicates soil properties, such water holding capacity, drainage and nutrient

supply. The texture of soil is determined by the percentage of sand, silt and clay which

formed the soil. For example, sandy soil contains 100% sand and sandy loam soil

contains 60% sand and 15% clay (McLaren and Cameron 1996). The soil texture

influences the rate at which water is supplied to plant roots. Generally, water supply to

plants is greater in a medium than in course or fine textured soil (Manrique 1992).

Sandy soil has a low water holding capacity therefore, potatoes grown in sandy soils

require daily irrigation to replace 100% of the daily evapotranspiration in order to

achieve maximum yield (i.e. approx. 60 t/ha). When the irrigation rates are lower than

90% replacement yield is reduced. Irrigation to replace 80% evapotranspiration reduces

yields from 60 t/ha to approximately 52 t/ha and this is further reduced to approximately

32 t/ha when irrigation replaces only 60% of the evapotranspiration. However,

34


excessive irrigation in sandy soil can cause nutrient leaching (Miller and Martin 1983).

Daily irrigation results in the highest yields (approx. 55 t/ha) and this is reduced to 50

t/ha when potatoes are irrigated every four days. Daily irrigation with interruption for

one week during tuber initiation reduced yield to 46 t/ha and interruption of irrigation

during tuber bulking further reduced the yield to 41 t/ha (Miller and Martin 1990).

In Western Australia the yield of potatoes grown in sandy soil in the Myalup area of

Western Australia is 45 t/ha whilst in sandy loam or gravely loams in the Manjimup

area is approximately 60 t/ha. These yields also vary with cultivar (Roger State,

pers.comm.).

2.8.5. Plant growth regulators

Tuberization in potato appears to be under hormonal control. Some of these hormones

act as promoters that increase the extent of tuberization and others as inhibitors which

delaying or prevent tuberization. Tuberization is inhibited by endogenous and

exogenous gibberellin application to plants grown under in-vitro, glasshouse and field

conditions. However, in other situations, exogenously applied GA promotes tuberization

through increased stem number. Anti-gibberellins that inhibit GA biosynthesis, such as

paclobutrazol and CCC, also promote tuberization. Other hormones such as auxins,

cytokinins, ABA and jasmonic acid are involved in the regulation of tuberization.

2.8.5.1. Endogenous gibberellins

Gibberellin is generally regarded as an inhibitor for tuberization. High levels of

endogenous gibberellins are generally associated with the inhibition, delay or

prevention of the tuberization process (Vreugdenhil and Struik 1989). Tuber initiation is

35


associated with declining endogenous gibberellins levels (Koda and Okazawa 1983a;

Xu et al. 1998b).

Endogenous GAs have been identified in different organs of potato plants. Immature

potato tubers contain GA20 and G15 (Xu et al. 1998b). Resting and developing buds

contain GA1, GA4, GA9 and GA20 (Jones et al. 1988: Xu, 1998b). Roots contain GA3,

GA8 and GA20 with GA3 being the most abundant (Abdala et al. 1995). Recently, all

organs of potato plants, including foliage, roots, stolons and tubers were found to

contain GA3 and GA1 (Abdala et al. 2002). .

The role of endogenous GA1 on tuberization was studied by examining concentrations

in developing stolons under tuber-inducing (basal medium with 8% sucrose) and non-

inducing (basal medium with 1% or 8% sucrose plus GA4/7) conditions. At the onset of

stolon formation the levels of GA1 increased more than 4-fold in stolons grown under

inducing compared with non- inducing conditions. During stolon elongation GA1

concentration usually remains high for plants grown under non-inducing conditions but

decreases considerably in plants grown under inducing conditions (Xu et al. 1998b).

The lowest concentration of GA1 is reached just before tuber initiation in the sub-apical

region of stolon tips. GA1 is the main regulator for tuberization and low GA1 levels are

a prerequisite for tuber initiation and enlargement (Xu et al. 1998b). At high GA

concentrations the transverse cortical microtubular cytoskeleton is stable and this allows

transverse cell division in the sub-apical region of stolon tips resulting in cell

elongation. At low GA concentrations the cortical microtubules are re-oriented to the

longitudinal or oblique direction and this allows for the enlargement of cells in the sub-

apical region of stolon tips thus resulting in tuber formation (Shibaoka 1993; Sanz et al.

1996).

In potato plants, shoot apices are the main sites of gibberellin synthesis (Menzel 1981)

and GA is transported basipetally (Menzel 1983). Paraquat + diquat mixture is sold

36


under the trade name of Spray.Seed®. This mixture is a commonly used herbicide to

desiccate potato haulms prior to tuber harvest (Summers 1980; Ashton and Monaco

1991). An approach requiring further investigation is the use of this herbicide at much

lower rates than harvest-desiccant rates in order to desiccate shoot apices and reduce

GA biosynthesis thus promoting tuberization.

All gibberellins in plants originate from ent-kaurene, which is a tetracyclic diterpene.

Ent-kaurene is derived from geranyl diphosphate aided by the activity of two enzymes,

namely copalyl diphosphate synthase and ent-kaurene synthase. Ent-kaurene is

converted to GA53 via GA12- aldehyde by cytochrome P450 enzymes followed by

metabolism to the bioactive GAs (Hedden 1997; Hedden and Proebsting 1999).

In shoots of potato the main metabolic pathway for GA synthesis is via the early-13-

hydroxilation pathway and most of the intermediates in this pathway, such as GA12-

aldehyde, GA53, GA44, GA19, GA20, GA29, GA1 and GA8 have been identified (Berg et

al. 1995). This GA pathway is common in the vegetative tissues of many other higher

plant species (Phinney 1984; Sponsel 1995). The non-hydroxylation pathway also

occurs in potato shoots as indicated by the presence of one of the pathway members,

namely GA51 (Berg et al. 1995).

The first reaction in both pathways is oxidation of GA12-aldehyde by monooxygenases

resulting in formation of GA53 and GA12. In the early-13-hydroxylation pathway, GA53

is further oxidized by dioxygenases resulting in formation of GA44, GA19, GA20, GA29,

GA1 and GA8. The final product, GA3 is derived from GA20 via GA5 (Hedden 1997).

All GA’s contain 19 C atoms and are biologically active in plants (Lange 1998), for

example, GA1 is a bioactive GA required for stem elongation. Regulation of active GA

forms occurs by the 3β-hydroxylation of GA20 to GA1. Both GA1 and GA3 have a 3β-

hydroxyl group, which confers high activity, and both are present in potato. Other GAs,

such as GA8, GA29, GA34 and GA51 are inactive due to the presence of a 2β-hydroxyl

37


group (Sponsel 1995). In the non-hydroxylation pathway, GA12 is oxidized by

dioxygenases to form GA15 followed by a series of oxidation reactions to form GA24

GA9 GA4, GA34 and GA7 (Hedden 1997). GA9 is also metabolized to GA 51 (Sponsel

1983).

2.8.5.2. Exogenous gibberellins

Applied GA has a considerable but variable impact on tuberization. The influence of

applied GA on tuberization depends on the developmental stage at which it is applied,

the application method and the GA concentration. There are conflicting reports on the

influence of GA application on tuberization (Vreugdenhil and Sergeeva 1999). Tuber

initiation can be delayed or inhibited (Krauss and Marschner 1982; Menzel 1983; Ewing

1985) but may also be promoted (Holmes et al. 1970; Marinus and Bodlaender 1978;

Bodlaender and van de Waart 1989). GA3 is commercially available in formulations that

are suitable for farming systems and GA3 is the most commonly applied GA.

The finding of greater number of tubers on a single plant after GA application is related

to increased numbers of stems (Holmes et al. 1970; Marinus and Bodlaender 1978;

Sekhon and Singh 1984; Mikitzel 1993). Underground stems are the sites of stolon

growth (Wurr et al. 1997) and stolons are the sites of tuber initiation (O' Brien et al.

1998; Xu et al. 1998a; Jacson 1999) thus, a plant with more stems has a greater number

of stolons that can initiate more tubers.

Increases in tuber number are also due to increased stolon branching, which is also

induced by GA application (Bodlaender and van de Waart 1989). Branched stolons can

also provide more sites for tubers to develop (Struik et al. 1988; Gill et al. 1989; Caldiz

1996). However, there is a lack of information about how stolon branching occurs with

GA application, how much this increases the number of tubers and whether any

differences between cultivars occur in field-grown potatoes. The lack of knowledge

38


about stolon branching is mainly because stolon number is very difficult to assess in the

field (Ewing and Struik 1992).

The number of tubers developed on GA-treated potatoes increases as stem number

increases (Holmes et al. 1970; Mikitzel 1993). In the cultivar Majestic, stem number

increased from 1.9 to 2.8 after GA was applied at 50 mg GA3/L and this increased the

number of tubers from 11 to 17 per plant (Holmes et al. 1970). In another cultivar,

Shepody, the stem number increased from 2 to 2.5 per plant after 1 mg GA3/L was

applied and tuber number increased from 5 to 6 per plant (Mikitzel 1993).

Only a limited range of GA concentrations are suitable for increasing the number of

tubers and their yield. Each potato cultivar requires a different GA concentration to

achieve maximum tuber production and yield. Outside the optimum concentrations,

tuber number and yield can be reduced. For example, when the cultivar Kufri

Chandramukhi was treated with 100 mg GA3/L the number of tubers developed per

plant was reduced from 13 to 9 (Sharma et al. 1998b). In fact, excessive GA

concentrations can deform the tubers (Bodlaender and van de Waart 1989; Struik et al.

1989b). This is probably related to the reduction in starch deposition at high GA

concentration (Mares et al. 1981; Sharma et al. 1998b). GA3 applied at 100 mg/L

decreases starch content in developing potato tubers by 13% (Sharma et al. 1998b). This

probably influences tuber development in stage C when small potatoes are increasing

their size and the stolon hook is straightened (Koda and Okazawa 1983a). When GA is

applied at high concentrations, the hook probably cannot straighten properly due to a

lack of starch to fill it.

The method of GA application is important in influencing tuber and whole plant

responses. Dipping seeds before planting is effective (Mikitzel 1993; van Ittersum and

Scholte 1993) and the uptake of GA is better with cut rather than whole seeds (Sekhon

and Singh 1984). Spraying seeds during storage (Holmes et al. 1970) and GA

39


application directly to the soil can also be effective (Struik et al. 1989b). Foliar sprays

of GA during plant growth give contrasting results. Sometimes GA application

promoted tuberization (Caldiz 1996; Caldiz et al. 1998) and in others cases GA

application inhibited tuberization (Lippert et al. 1958; Lovell and Booth 1967; Sharma

et al. 1998b). These contradicting results are likely related to the concentrations used

and the frequency and time of application. High concentrations of GA at multiple

applications (Sharma et al. 1998b) and later stages of tuber development (Bodlaender

and van de Waart 1989) frequently inhibit tuberization.

Each tuber progresses through several developmental stages and the stage at which GA

is applied is crucial for determining the resulting size distribution of tubers at harvest.

Application of GA prior to planting is generally effective in increasing yield of small

tubers and reducing the yield of large tubers (Sekhon and Singh 1984; Mikitzel 1993;

van Ittersum and Scholte 1993). Application of GA after shoots have emerged is

generally more variable (Bodlaender and van de Waart 1989; Struik et al. 1989b;

Sharma et al. 1998b). Although stolonization is GA dependent, tuber initiation and

tuber enlargement are inhibited by GA. Ideally, GA levels should be high enough to

increase the number of stolons developed and then decline to promote tuber formation.

Applied GA3 promotes shoot and stolon growth (Sharma et al. 1998b). Plants are taller

due to increased length and number of internodes and have a higher shoot dry weight

(Menzel 1980; Sharma et al. 1998b). Stolon length is increased in GA treated plants and

can be twice that of untreated plants (Sharma et al. 1998b). The number of stolons can

also be increased after GA application (Holmes et al. 1970).

Potato cultivars, such as Pontiac, Keswick, Kennebec, Shepody and Spunta naturally

produce many large tubers (Timm et al. 1962; Smeltzer and Mackay 1963; Mikitzel

1993; Caldiz 1996). This is undesirable for seed potato production because of the risk of

seed pieces without eyes when large tubers are cut (Neilson et al. 1989) and in the

40


tropics, a higher rate of seed disease. Whole seeds are too large to use as a commercial

means of plant propagation and so reducing tuber size is necessary to reduce seed costs.

This is convenient since seeds are sold by the tonne, regardless of tuber size. The use of

small seeds has considerable advantage both economically and for the reduction of

disease. There are also benefits in terms of seed storing, handling and planting (Allen

and O'Brien 1987). Under some circumstances the application of GA to cultivars that

naturally produce large tubers has been shown to increase the number of tubers and

reduce their average size (Holmes et al. 1970; Mikitzel 1993).

Generally, GA has no carry-over effects on the growth of subsequent crops (Holmes et

al. 1970; Bodlaender and van de Waart 1989). Carry-over effects of GA on field-grown

progeny in the year following GA application had no effect on shoot emergence and

growth. In fact, tuber yield in the progeny of the GA treated plants was the same as the

control (Bodlaender and van de Waart 1989). This implies that there are no constraints

for the use of GA in seed potato production. GA3 occurs naturally in potatoes (Abdala et

al. 2002) and therefore progeny tubers should not be harmful for human consumption.

When GA is applied before planting it induces early shoot emergence (Dyson 1965;

Holmes et al. 1970; Marinus and Bodlaender 1978). In the cultivar Majestic, shoot

emergence was accelerated by 5 and 10 days when 50 and 100 mg GA3/L was applied.

The early emergence was correlated with a more rapid rate of cell elongation that was

presumably induced by GA application (Metraux 1987; Jacobsen et al. 1995; Sanz et al.

1996). Gibberellin reduces apical dominance (Bishop and Timm 1968; Holmes et al.

1970) and allows the development of multiple sprouts which produce more stems.

Gibberellin application reduces chlorophyll content of potato leaves (Agarwal et al.

1983; Sharma et al. 1998b) probably due to an influence of GA on chlorophyll

biosynthesis (Mathis et al. 1989; Jacson and Prat 1996) and increases leaf area (Wheeler

and Humphries 1963).

41


2.8.5.3. Abscisic acid (ABA)

A high concentration of endogenous ABA is associated with promotion of tuberization.

Changes in endogenous ABA concentration during tuberization are opposite to that of

GA. In elongating stolon tips (Stage A), ABA levels are extremely low and begin to

increase when stolon tips start to swell (Stage B). The ABA level continues to increase

until tuberization is completed (Koda and Okazawa 1983a). The tuberization frequency

in non-inducing media (agar + sucrose + GA4/7) without ABA decreased as sucrose

content in the media decreased. Addition of ABA resulted in 100% tuberization

frequency. This implies that ABA promotes tuberization by counteracting GA (Xu et al.

1998b).

2.8.5.4. Auxin

The levels of endogenous auxin in developing stolons do not change much during

tuberization and generally remain quite low (Koda and Okazawa 1983a). However,

there is a positive correlation between tuber growth rates and auxin content and

removing the fastest growing tubers increases the growth rate of other individual tubers,

indicating that auxin may exert its influence by altering sink strength (Marschner et al.

1984).

2.8.5.5. Cytokinins

Cytokinin promotes tuberization during the later stages and probably exerts its influence

by altering the rates of cell division (Jameson et al. 1985). In elongating stolon tips the

level of endogenous cytokinin is low and then increases as stolon tips begin to swell.

The highest levels of cytokinins are found in the fully swelled stolons and thereafter it

decreases slightly (Koda and Okazawa 1983a). This implies that cytokinin promotes

tuberization. In another experiment however, the levels of endogenous cytokinin at

42


tuber initiation (swelling of stolon tips) were low and increased at the later stages

leading to the conclusion that cytokinins were not associated with tuber initiation;

instead they act on cell division (Jameson et al. 1985).

2.8.5.6. Jasmonic acid (JA)

Tuberization in potato is induced by a stimulus perceived by the leaves and translocated

to stolons (Ewing 1981). The chemical nature of the stimulus remained unknown until

1988 but has been investigated since 1956. The stimuli initially appeared to be a couple

of acidic substances. The first substance was soluble in ethyl acetate but hardly soluble

in water and the second had the opposite properties (Koda and Okazawa 1988).

Isolation of these substances in potato leaves showed that they were tuberonic acid

glucosides, which are closely related to JA (Yoshihara et al. 1989).

In studies conducted in controlled environments and in the field, JA was a very active

hormone that induced tuberization in vitro (Pelacho and Mingo-Castel 1991; Abdala et

al. 1996; Abdala et al. 2002). Endogenous JA occurs in foliage (Koda and Okazawa

1988; Koda et al. 1988; Helder et al. 1993b), roots, stolons and tuber periderms. Tuber

periderms have the highest content over others (Abdala et al. 1996). In field–grown

potatoes, endogenous JA content during tuberization changed over time. During stolon

elongation the JA content in stolon tips was high and then decreased considerably

during swelling of the stolon tips. Thereafter JA levels increased dramatically in

swollen stolons and then decreased again in young tubers (Abdala et al. 2002). The high

endogenous JA content in swollen stolons probably caused cell expansion in the

perimedullary region of the tuber (Takahasi et al. 1994).

43


2.8.5.7. Paclobutrazol

Paclobutrazol is a growth retardant, which blocks gibberellin biosynthesis by inhibiting

microsomal oxidation of ent-kaurene, kaurenol and kaurenal (Davis et al. 1998).

Paclobutrazol is commonly used to reduce vegetative growth and increase the yield in

fruit trees such as apple, peach and cherry. The increased yield is thought to result from

the greater numbers of flowers developed after treatment with paclobutrazol (Edgerton

1986).

In potato, paclobutrazol promotes tuberization in tissue culture (Harvey et al. 1992;

Simko 1993, 1994) and pot culture (Balamani and Poovaiah 1985; Bandara and Tanino

1995; Bandara et al. 1998). In pot grown potato paclobutrazol is applied at early stolon

initiation (Bandara et al. 1998; Bandara and Tanino, 1995). Increased numbers of tubers

after paclobutrazol application, probably occurred as a result of blocking GA

biosynthesis resulting in low levels of endogenous GA, which is necessary for tuber

initiation (Ewing 1990; Ewing 1995; Xu et al. 1998b). Paclobutrazol increased tuber

number from 1.6 to 7 per plant in Norland potatoes grown in pots (Bandara et al. 1998).

However, the use of paclobutrazol in field-grown potatoes has not been investigated. If

paclobutrazol has the same influence on tuberization in the field, as in tissue and pot

culture, it would be potentially beneficial for seed potato production.

Growth retardants that influence assimilate partitioning generally favor tuberization. In

tissue culture, paclobutrazol diverts large amounts of assimilates to tubers (Simko 1991)

and other storage organs such as corms (Ziv 1989; Steinitz et al. 1991). Chlorocholine

chloride (CCC) (a growth retardant) also diverts more assimilates to tubers in potatoes

(Sharma et al. 1998a; Sharma et al. 1998b). In developing potato tubers, CCC

application increases the activity of enzymes involved in sucrose metabolism (i.e.

sucrose synthase and sucrose phosphatase), resulting in high starch production for tuber

44


filling (Sharma et al. 1998a). Although not certain, a similar mechanism may also

operate in paclobutrazol-treated plants.

Application of paclobutrazol to potato plants grown in pots and in tissue culture

suppresses shoot and stolon growth. Plant height decreases due to a reduction in

internode length. Shoot fresh and dry weight, stolon + root fresh and dry weight

decreases with paclobutrazol (Balamani and Poovaiah 1985). Suppression of shoot and

stolon growth with paclobutrzol is probably due to reduction in endogenous GA via a

block in GA biosynthesis (Davis et al. 1998). GA is an important regulator for cell

elongation (Xu et al. 1998b) and reducing GA levels would inhibit cell elongation

resulting in shorter organs.

The effectiveness of paclobutrazol application varies with its application at different

stages of tuber development. Paclobutrazol application during early stolon initiation

increased the number of tubers from 1.6 to 7 per plant but when applied at late stolon

initiation there was a slightly less pronounced increase in the number (from 1.6 to 5.3)

of tubers developed (Bandara et al. 1998). Unfortunately, early and late stolon

initiations were not clearly defined in these studies. However, paclobutrazol should be

applied early, just before tuber initiation begins. This is because, at tuber initiation, the

endogenous GA concentrations need to be low (Xu et al. 1998b) and application of

paclobutrazol should thus reduce GA synthesis and lower GA concentrations (Davis et

al. 1998; Simko 1993).

In roots of potato grown in pots or from shoot cuttings CCC application reduced

endogenous GA3 concentration and doubled the number of cuttings bearing tubers 26

days after planting. It was concluded that endogenous GA3 delays tuberization (Abdala

et al. 1995). Paclobutrazol inhibits GA biosynthesis by blocking metabolism from ent-

kaurene to ent-kaurenoic acid (Rademacher 1999). Combining pre-planting GA3 with

post planting paclobutrazol applied at early tuber initiation may give mutually beneficial

45


responses. The GA3 might increase stem number, stolon number and sites for tuber

initiation and paclobutrazol could also promote tuber initiation by lowering endogenous

GA’s levels and redirecting assimilates toward tubers to ensure their growth. This is

potentially a very powerful combination to enhance seed potato production but it has

not been investigated and requires study.

2.9. Determining time of tuber initiation

Determining the time of tuber initiation under the field conditions is done by repeated

sequential harvesting of plants and measuring tuber number and size distribution

(Demagante and Van der Zaag 1988; Barry et al. 1990; Firman et al. 1991; O' Brien et

al. 1998). The use of tuber size as a criterion for determining tuber growth stage is

commonly used in the field. The length of the longest tubers (i.e. 2, 10, 25, 35 and 50

mm) refers to the stages (S) of their development (i.e. S1, S2, S3, S4 and S5 respectively)

(Williams and Maier 1990a; Williams and Maier 1990b). In the present experimental

investigations, early tuber initiation was defined as the time when 50% of the stolon tips

were swollen to twice the stolon diameter (proximal from the tip) (Demagante and Van

der Zaag 1988). This can also be referred to as stage B of stolon development (Koda

and Okazawa 1983a). The use of this criterion is appropriate here because swelling of

stolons on a single plant does not occur in synchrony. (Struik et al. 1991). Late-tuber

initiation is defined as the time in development when the length of the longest tuber is

10 mm (S2) (Williams and Maier 1990a; Williams and Maier 1990b) or also as stage C

during stolon development (Koda and Okazawa 1983a). When plants are grown in

tissue culture tuber initiation is defined as the time when the first sign of swelling of

tubers is observed in explants (Sergeeva et al. 2000). This approach is much simpler

and easier compared with studies on field-grown potatoes because tubers can be easily

46


seen in the clear container and agar media. In addition, it does not require destructive

harvests as in the case of field-grown potatoes.

2.10. Factors influencing tuber-size distribution

Production of potatoes with certain features considered to be optimal for particular

outlets requires an understanding of factors which influence the development of tuber

number and size distribution (Struik et al. 1990). In terms of seed potato production, the

primary aim is directed toward maximizing the development of high proportions of

small tubers.

2.10.1. Number of plants per unit area

The number of plants per unit area, or plant density, is determined by the number of

seeds planted (Struik et al. 1990). In Tasmania, a density of 16 Atlantic plants/m2 is the

most economic density for optimum yield of small round seeds (31-60 mm) (Laurence

et al. 2000). In Victoria, 7 to10 Atlantic plants/m2 is the optimum density for the yield

of 35-110 g potatoes (Henderson et al. 2000).

2.10.2. Number of stems

Main stems, which grow directly from seed tubers, are regarded as units of plant density

in potato production (Allen and Wurr 1992) and their numbers are the most important

factor determining the number of tubers and their size distribution (Bleasdale 1965).

Stem number is influenced by many factors, such as seed size and exogenous GA

application. Whole seeds produce more stems per plant than cut seeds (O'Brien and

Allen 1992). Small seeds (29-32 mm) produce 69,000 stems/ha, medium seeds (35-41

mm) produce 152,000 stems/ha and large seeds (48-54 mm) produce 270,000 stem/ha.

Tuber number increases with increasing stem number (Allen and Wurr 1992) and

47


application of GA3 prior to planting leads to increased numbers of stems and tubers

(Mikitzel 1993).

2.10.3. Number of tubers

Tubers are initiated in the sub-apical region of stolons (Koda and Okazawa 1983a; Xu et

al. 1998a; Jacson 1999) and stolons grow from underground stems (Wurr et al. 1997).

Potato cultivars can have the same number of stolons but have different numbers of

tubers. Tubers are formed on stolons, which develop at different nodes along the axis of

the belowground stems. The more tubers that are formed from stolons developed at a

single node, the greater the competition for assimilates amongst the nodes. Tuber size

distribution is influenced by the position of tuber bearing stolons on the stem nodes,

because this influences the growth rates of the tubers. The first tuber formed is usually

on the lowest stem node (basal node), which is the closest node to the seed tuber.

Growth rates of tubers arising from stolons in basal nodes are higher than that from

stolons at upper nodes (Cother and Cullis 1985). Tuber size decreases as the distance

increases from the basal node (Gray and Smith 1973). The difference in growth rates is

related to differences in rates of cell division (Gray 1972). Thus, the position of nodes at

which tubers are formed and the competition amongst nodes for assimilates influences

tuber size distribution (Wurr 1977).

Stolon number also influences tuber number. Generally, the more stolons developed on

a plant, the more tubers that are produced (Haverkort et al. 1990b; Haverkort et al.

1990c). Although primary stolons are the most important tuber bearing site (Oparka

1987), the tubers developed on secondary and branch stolons also contributes to tuber

numbers (Wurr et al. 1997; Bodlaender and van de Waart 1989; Gill et al. 1989). Stolon

branching is promoted by several conditions including a long photoperiod (Struik et al.

48


1988), applied gibberellic acid (Bodlaender and van de Waart 1989), high temperature

(Struik et al. 1989a) and drought (Struik and van Voorst 1986).

The growth rate of individual tubers on a plant varies. At any given time only a few

tubers grow rapidly to a maximum size and then their growth rate declines. This is

followed by the rapid growth of other tubers (Moorby 1967; Moorby 1968). The largest

tuber at any given time is not necessarily the largest at harvest (Moorby 1968; Ahmed

and Sagar 1981). The rate of growth during early tuber growth is determined by factors

outside the sink including the characteristics of the stolon prior to tuber initiation. The

growth rate of individual tubers increases as the diameter and volume of stolons

increases. In the later stage of development, when tubers have established their own

sink strength, growth rates of individual tubers are mainly regulated by factors within

the sink, such as sink size and sink activity (Engels and Marschener 1986).

The pattern of assimilate movement in potato plants also influences tuber size. In terms

of tuber position relative to the source leaves, more assimilate moves from source leaves

to tubers that grow on the same side of the stem (Gray and Smith 1973). Thus

assimilates follow the most direct vascular pathways. However, when a stronger sink for

assimilates is located on the opposite side of source leaves, some redirection of

assimilates occurs. The assimilates from a single stem are translocated only to tubers

directly attached to that stem. There is virtually no translocation of assimilates to tubers

on separate stems (Oparka and Davies 1985). However, assimilate translocation and

transfer can occur between tubers on the same stem (Moorby 1970).

2.11. Conclusion

The project aims at increasing tuber number and yield of small tubers weihing 20-55 g

Some approaches will be done including application of plant growth regulators,

treatments that reduces apical dominance, application of low doses of herbicide and

49


storage duration at 4oC. Amongs plant growth regulators GA3 is known to increase stem

number and this in turns increases tuber number. High tuber number will induce

competition between tubers for assimilates and this will results in small tubers. For

these purposes cut seeds potatoes will be dipped in GA3 solutions one day before

planting. A growth retardant paclobutrazol is known can redirect assimilates from

shoots to tubers and this promotes tuber initiation in experiments conducted in pots and

tissue cultures. The information about the use of paclobutrazol under field conditions is

lacking. In this project paclobutrazol will be applied as a foliar spray at early tuber

initiation and this approch is expected to increase tuber number via redirecting

assimilates to tubers. Combining GA3 and paclobutrazol may also give mutual combine

effect where GA3 is expected to increas stem number and tuber number and

paclobutrazol is expected to redirect assimilates from shoot to tubers with overall effects

benefits tuberization. Gibberellin will be applied to seed tuber before planting and then

paclobutrazol will be applied as a foliar spray at early tuber initiation. Another problem

in increasing tuber number via increasing stem number is apical dominance where the

outgrowth of lateral sprouts is inhibited by apical sprouts. Reducing apical dominance

is important for increasing stem number which in turn increases tuber number. There are

some methods which can be used, such as physical treatments including apical sprout

removal and chemical treatment including application of carvone and herbicide (Spray

Seed®). All these treatments remove the source of inhibition of lateral sprout growth in

apical sprouts.

An appropriate physiological age of seeds is required for maximum sprout and stem

number. Certain physiological age will be created by storing seeds at 4oC for a period of

time (e.g. weeks) and changes in sprout and stem growth will be observed in glass

house experiments

50


51

Chapter 3 Manipulation of apical dominance

Chapter 3

Manipulation of apical dominance by chemical and physical

treatments leads to increased tuberization in potato (Solanum

tuberosum L.) varieties Atlantic and Granola

3.1. Introduction

In Western Australia, the seed potato varieties Atlantic and Granola are important for

export to Southeast Asia, especially Indonesia. This market requires small round seed

potatoes (20-55 g). Seeds are rarely cut in the tropics because of high humidity and high

temperatures, which increase the likelihood of diseases infecting the wound (Batt 1997).

Compared with potato plants having only one or two stems, those with several stems

generally develop more tubers (Mikitzel 1993). Greater numbers of tubers lead to

greater competition for resources between tubers resulting in smaller tubers (Moorby

1967; Bishop and Timm 1968).

Clearly, apical dominance is an important factor determining sprout numbers in potato.

It is defined as the suppression of lateral sprout growth by a single (or multiple) apical

sprout(s). The suppression of lateral sprout growth can occur between sprouts at a single

eye or extend to the suppression of sprout growth from sprouts located at other eyes on

the seed tuber (Kumar and Knowles 1993). Thus lateral sprouts are subjected to

correlative inhibition by apical sprouts (Phillips 1969). The importance of apical

dominance is illustrated in many crop species and for example, in sorghum, the release

of apical dominance leads to the development of more reproductive shoots thus

resulting in greater yields (Isbell and Morgan 1982). This is also a common occurrence

in tomato and tobacco plants and also in fruit trees (Martin 1987).

Auxin produced by the apical bud may directly inhibit the outgrowth of lateral buds

(Knowles et al. 1985; Kumar and Knowles 1993). The mode of auxin action is not

52


certain and auxin may induce the production of secondary compounds that in turn

inhibit lateral bud growth (Tamas 1987; Cline 1994). Cytokinins may also be involved

in reducing apical dominance (Staden and Dimalla 1978; Wang and Wareing 1979;

Bangerth et al. 2000). Apical dominance tends to decline with seed age. Older seed

produces more stems than younger ones and each variety has an optimum tuber age that

yields the maximal numbers of stems (van der Zaag and van Loon 1987). This is

probably due to a higher capacity for auxin catabolism in the older seeds and their

reduced ability to transport auxin basipetally (Kumar and Knowles 1993).

Various physical and chemical treatments have been used to reduce apical dominance in

potato. Physical treatments include the removal of apical sprouts and dissecting out the

apical complex (Hay and Hampson 1991). In the potato variety Atlantic, removing

apical sprouts increased the number of tubers developed by plants and also reduced the

average tuber weight without reducing total tuber yield. These physical treatments are

very effective for altering the production of small seed potatoes for export (Harrington

2000) but they are not very practical for large scale production.

In addition to the physical removal of apical buds, several chemicals can also be used to

damage apical sprouts thus suppressing sprout growth during storage. As with physical

sprout removal, chemical application reduces apical dominance. Isoprophyl N-

(3chlorophenyl) carbamate (CIPC) is the most commonly used sprout suppressant in

many countries (Prange et al. 1997). It has been the major sprout suppressant in

Australia since the mid 1960’s (Baker 1995). Other chemicals, such as isopropyl N-

phenylcarbamate (IPC), maleic hydrazide (MH) and tecnazene (1,2,4,5-tetrachloro-3-

nitro-nitro benzene) are also commonly used in many countries. Due to concerns over

chemical residues, there continues to be an effort worldwide to find alternative sprout

suppressants (Kerstholt et al. 1997; Prange et al. 1997).

53


The search for alternative sprout suppressants has led to research on natural products.

One of these is the monoterpene, S-carvone, which is a volatile essential oil, extracted

from caraway (Carum carvi L.) seed. Carvone is traded under the name Talent® and

was released in 1994 in the Netherlands (Wiltshire and Cobb 1996; Hartmans et al.

1995). Dimethylnaphthalene (DMN) is another sprout suppressant that naturally occurs

in potato tubers (Burton et al. 1992). Carvone and DMN can be used as sprout

suppressants during storage. The effect of these sprout suppressants depends on the

variety, the timing and the rate of applications (Beveridge et al. 1981b; Hartmans and

Oosterhaven 1998).

The use of sprout suppressants like carvone (S-carvone) and DMN (1,4-

dimethylnaphthalene) for seed potato production is relatively new. Carvone damages

primordial sprouts and so it can be used to reduce apical dominance and this allows

enhanced lateral sprout growth (Baker et al. 2002) and branching of main sprouts. This

indicates that carvone reduces apical dominance (Oosterhaven et al. 1995). In fact,

carvone is as effective as manual desprouting for stimulating sprout and stem growth in

the Dutch variety, Bintje (Hartmans and Oosterhaven 1998). If similar results could be

attained with Atlantic and Granola it would provide a practical option to manual

desprouting for seed potato production. In terms of the influence of carvone on tuber

growth, studies with Russet Burbank potatoes have shown that carvone promoted

multiple stem development (Brown et al. 2000). In this variety, the tuber number

increased and a greater proportion were small tubers (30-59 g). Dimethylnaphthalene

(DMN) has also been shown to increase tuber number and shift the yield toward a

greater proportion of small tubers without reducing total yield. The influence of DMN is

variety specific and a greater effect of DMN has been noted on Ranger Russet than

Russet Burbank (Haines et al. 2002). The response of Atlantic and Granola to applied

carvone has not been examined.

54


A commercial herbicide mix containing paraquat and diquat is sold under the trade

name of Spray.Seed®. It is registered as a pre-, and at emergence, knockdown herbicide

for annual grasses and broadleaf weeds in cultivated crops including potato. The

recommended rate for potato is 3.5 L Spray.Seed®/ha (Parsons 1992). Paraquat and

diquat are bipyridynium herbicides of group ‘L’ with a mode of action aimed at the

inhibition of photosynthetic reaction in photosystem I (Summers 1980). Rapid

desiccation of foliage can be observed within a few hours of application and these

herbicides are frequently used as a pre-harvest desiccant in root crops, such as potato

(Summers 1980; Ashton and Monaco 1991). At lower rates than recommended for

killing weeds, Spray.Seed®, causes less damage but the correct timing and rate of

applications need to critically determined. Young shoots are the most susceptible and

death of shoot tips may redirect assimilates away from new shoot growth towards

tubers. This may also reduce gibberellin levels in plants because young shoots are sites

of gibberellin biosynthesis in potatoes (Menzel 1981). Reduced gibberellin transport to

stolons would result in low levels of gibberellin in stolon tips, which promotes

tuberization (Krauss 1981; Krauss and Marschner 1982; Koda and Okazawa, 1983; Xu

et al. 1998b).

It has been proposed that strong apical dominance in the potato variety Atlantic limits

stem number and hence the production of small tubers (Harrington 2000). Granola

produces a higher proportion of small tubers than Atlantic but information about its

apical dominance is lacking. In this study, the apical dominance of these contrasting

varieties was manipulated in order to test the hypothesis that increased stem number

leads to the development of greater numbers of (small) tubers.

The aim of experiments was to increase yield of small round seeds weighing 20-55 g

without reducing total tuber yield. The hypotheses tested were that apical sprout

removal, carvone application, and increased storage duration at 4oC would reduce apical

55


dominance and increase stem number. This in turn, should increase tuber number and

induce high competition among tubers for assimilates thereby reducing average tuber

size without reducing total yield. Paraquat and diquat damage shoot tips and the death

of young shoot tips may have two effects. It may redirect assimilates away from new

shoots to tubers and it may reduce GA biosynthesis and transport to stolons thereby

further promoting tuber initiation.

3.2. Materials and methods

Pot trials

Potato plants in Experiment 1 (Figure 3.1) were grown in a glasshouse phytotron

(22oC/18oC, day/night) at the University of Western Australia from 25 August to 2

November 2001. Seeds of Atlantic and Granola held in cold storage (4oC) (same source

as experiment 2 and 3) were removed every two weeks during weeks 22 to 30. After

seeds were removed from storage they were placed at 20oC for 2 weeks and sprout

number per seed was recorded. For each variety, 5 potato tubers (each 150-200 g) were

cut into 4 pieces (rose end to heel end) without damaging the eyes (20 pieces) and

another five tubers were left intact. Seed pieces were dusted with Tatodust (2 g

Mancozeb®/ kg) to control Fusarium and both whole and seed pieces were treated with

toloclofos-methyl (2 g Rizolex®/kg) to control tuber-borne Rhizoctonia just prior to

planting. Seed tubers were planted in 1.5 L pots filled with potting mix.

A completely randomized design was used for the pot trial. Fifty pots of two varieties

with whole and cut seeds were randomized on the bench and they were watered every 3

days. The number of sprouts on intact and cut tubers was counted 2 days prior to

planting. The time from planting to complete shoot emergence was recorded and plants

were harvested 30 DAP. At harvest the stem number per plant and plant height was

measured.

56


Figure 3.1. Potatoes grown in a glass house phytotron at the University of Western Australia to investigate the influence of seed storage duration (22, 24, 26, 28 and 30 weeks) at 4oC on sprout number, time of emergence, stem number and plant height for gowing period of 30 days. Analysis of variance was carried out using Genstat 6 (Lawes Agricultural Trust,

Rothamsted Experimental Station) for a completely randomized design. Differences

between the treatment means were compared using the least significant differences at P

= 0.05

Field Experiments

Site characteristics. Experiments 2 and 3 (Figure 3.2) were conducted at the Western

Australian Department of Agriculture, Manjimup Horticultural Research Institute

(34o18’S, 116o 7’E) in a field of sandy loam soil from October 2001 to March 2002.

Chemical and physical characteristics of the top 15 cm of soil are presented in Table

3.1.

57


Experiment 2 Experiment 3

Figure 3.2. Experiment 2 (paraquat + diquat and apical sprout removal treatments) and Experiment 3 (carvone treatment) in the field at Manjimup Horticultural Research Institute, Western Australia Department of Agriculture.

Table 3.1. Chemical and physical characteristics of top 15 cm of soil collected before experiments began for Experiment 2 and 3.

Parameter Unit Experiment 2 Experiment 3pH CaCl2 - 5.8 5.5 P mg/kg 86 135 K mg/kg 85 79 NO3-N mg/kg 12 13 NH4-N mg/kg 3 4 Total N % 0.16 0.13 Organic C % 3.19 2.7 Texture Sandy loam Sandy loam Colour Brown Brown

P and K were measured as per Colwell (1963), NO3-N as per Reardon et al., (1966), NH4 as per Best (1976).

Crop management. Certified seeds of Solanum tuberosum L. cvs. Atlantic and Granola

were used. Seeds were stored at 4oC for 6 months and removed from cool storage 2

weeks before planting for sprouting. Potatoes were hand cut (50 ± 5 g) with at least two

eyes per tuber segment and were dusted immediately with Tatodust (2 kg Mancozeb®/t)

and toloclofos-methyl (2 kg Rizolex®/t) as above. Experiments 2 and 3 were sown on 24

and 25 October 2001 using a single-row, tractor-mounted planter with within row

58


spacing of 0.15 m (88, 333 plants/ha). The site was hilled when emerged shoots were

about 10 cm tall (13 DAP).

Weeds were controlled by applying metribuzin (500 mL Sencor®/ha for Atlantic and

1.11 mL Sencor®/ha for Granola) and paraquat dichloride (3 L Gramoxone®/ha) when

they emerged. Crops were protected against potato moth (Phthorimaea operculella) and

green peach aphid (Myzus persicae) by applying methamidophos (700 mL Nitofol®/ha)

and permethrin (200 mL Ambush®/ha). Rotation sprays of difenoconazole (200 mL

Score®/ha) and chlorotalonil (2 L Bravo®/ha) were applied regularly to protect plants

against potato diseases such as target spot (Alternaria solani).

Irrigation was applied using impact sprinklers at 220 kPa and was shedulled with

tensiometer as recommended by Hegney and Hoffman (1991). Tensiometers were

installed at 15, 30 and 50 cm depth shortly after shoot emergence. Irrigation was given

when tensiometer readings fell to 20 cb at 30 cm depth by applying 20 mm of water to

recharge the crop root zone.

Fertilizers were applied according to standard commercial practice. Potassium (82 kg

K/ha as K2SO4) was broadcast and incorporated one week before planting. Other

fertilizers, trace elements and magnesium were banded at the time of sowing. The trace

element plus magnesium mixed contained (kg/ha) Mg (13), Zn (7), Mn (7), S (7), Fe

(2), Cu (2), B (0.6) and Mo (56) (g/ha). Total phosphorus (P) and potassium (K) were

applied at 250 and 311 kg/ha. Nitrogen was applied (as NH4NO3) at sowing and again at

100% crop emergence at a rate of 100 kg/ha at each application.

Treatments. Experiment 2 had two treatments. The first treatment was the influence of

apical sprout removal. Apical spouts were defined as the largest and longest sprout

emerged from the rose end of tubers and these were removed by hand using sterile

forceps. Seeds were left at room temperature (20 ± 5)oC for one week before planting.

The second treatment was application of a mixture of paraquat and diquat Spray.Seed®

59


(125 g/L paraquat as paraquat dichloride, 75 g/L diquat as diquat dibromide

monohydrate, Crop Care Australasia Pty Ltd Pinkenba, QLD) using a boom spray at

three rates of Spray.Seed® (0, 250 and 500 mL/ha). The rate of 250 mL/ha is referred to

as a very low rate and 500 mL/ha is referred to as a low rate. Paraquat and diquat was

applied at two times as a foliar spray, during early tuber inititation (41 and 44 DAP for

Atlantic and Granola respectively) and again during late-tuber initiation (48 DAP for

both Atlantic and Granola). Early tuber initiation was defined as the time when tuber

diameter was twice that of the stolon (Firman et al. 1991) and is the same as stage B of

stolon development (Koda and Okazawa 1983). Late tuber initiation was defined as the

stage of tuber development when the longest tuber was 10 mm or stage C stolon

development (Koda and Okazawa 1983).

In Experiment 3, carvone (S-Carvone, C10H14O, Range Products, China) was applied to

sprouted seed tubers at a concentration of 0.6 mL/kg. Carvone (84 mL) was mixed with

water (500 mL) in a fry pan and placed under closed wire racks with a waterproof

tarpaulin and the rack was filled with seed potatoes (70 kg/ variety). Carvone was

simmered for 1 hour 45 minutes to allow sufficient time for the vapors to permeate the

tubers and desiccate sprouts. Controls were treated with water only with the same

procedures as carvone application.

Experimental design. In Experiments 2 and 3 a split plot design was used with the two

varieties as the main plot randomized within each block with 4 replications. In

Experiment 2, there were 3 rates of paraquat + diquat and two applications times (early

tuber initiation and late tuber initiation) factorized as sub plots. Apical shoot removal

replaced the second control. For both experiments 2 and 3 main plot size was 2.4 m

wide (3 rows) by 30 m long. Sub plots were 2.4 m wide (3 rows) by 5 m long.

60


Plant measurements. In Experiment 2 the time of first and complete emergence was

recorded. Three plants were sampled on each sub plot from buffer rows 82 DAP to

measure stem number per plant, chlorophyll content of leaves and shoot dry weight. In

Experiment 3, the time of first and complete emergence was recorded. Plants were

sampled as above 74 DAP to measure stem number. Experiments 2 and 3 were

mechanically harvested on 19 (146 DAP) and 20 (147 DAP) of March 2002

respectively, after the haulms were killed using paraquat + diquat (3 L Spray.Seed®/ha)

2 weeks before harvest. Potatoes from 5 m of the central rows were graded into

categories (<20, 20-55, 56-110, 111-200, 201-250 and >250 g). The number of tubers

and their weights for each category were counted. Diseased and machine-damaged

tubers were discarded.

Chlorophyll analysis. Three fully expanded leaves were harvested per plant and they

were wrapped in aluminum foil, frozen and transported in a car freezer from the field to

the laboratory. Leaves were freeze dried for 3 days and then ground with a ball mill

grinder to a fine powder. Leaf powder (10 mg) was placed in a 1.5 ml Eppendorf

centrifuge tube with cold absolute methanol (1.25 ml). Tubes were shaken for 45

minutes at 4oC in the dark and then centrifuged at 5000 rpm for 5 minutes. An aliquot

(0.5 ml) was transferred to a cuvette and diluted with cold absolute methanol (1 ml).

Absorbance was measured at two wavelengths (652.4 nm, and 665.2 nm) using a

Shimadzu UV-1601 Spectrophotometer. Chlorophyll a and b contents were calculated

according to Wellburn (1994).

Chlorophyll a = 16.72A665.2nm - 9.16A652.4nm

Chlorophyll b = 34.09A652.4nm - 15.28A665.2nm

61


where A665.2 and A652.4 were absorbance at 665.2 nm and 652.4 respectively.

Chlorophyll concentrations derived from these equations were expressed as mg/g dry

weight.

Statistical analysis. Analysis of variance for the data was determined using Genstat 6

(Lawes Agricultural Trust, Rothamsted Experimental Station) for split plot design.

Differences between treatment means were compared using least significant differences

at P = 0.05.

3.3. Results

Pot trial

Sprout number per seed tuber. Generally, sprout number increased with tuber storage

duration and Granola produced more sprouts than Atlantic. After a prolonged storage

period most sprouts emerged from lateral eyes. The relationship between storage

duration and sprout number was linear in both Atlantic (y = -3.64 + 0.25x, r2 = 0.99)

and Granola (y = -3.07 + 0.25x, r2 = 0.86) (Figure 3.3). In Atlantic the sprout number

increased from 1.9 (after 22 weeks in cool store) to 2.4 (after 26 weeks) and had

doubled after 30 weeks. In Granola, sprout number increased from 2.3 (22 weeks) to 3

and 4.1 after 24 and 26 weeks storage period respectively. This number of sprouts

remained stable afterwards from week 28 to 30.

Emergence. Generally, the time of complete shoot emergence was hastened by

increasing storage duration at 4oC and cut seeds emerged earlier than whole seeds. The

earliest shoot emergence was observed after 28 weeks storage (Figure 3.4). In whole

and cut seeds of Atlantic, that had received 22 weeks cold storage, shoots emerged

about 15 DAP. In tubers that received 28 weeks cold storage the emergence time was

62


reduced to 10 DAP (cut seeds) and 12 DAP (whole seeds). The emergences for both cut

and whole seed was 11 DAP when tubers had been stored for 30 weeks. In contrast,

there was less influence of storage duration on sprout emergence in Granola using

whole seeds. The earliest shoot emergence from

whole seeds stored for 28 weeks was found to occur 9 DAP whilst shoot emergence

from cut seeds was very fast (5 DAP).

Storage duration (weeks) at 4oC

22 24 26 28 30

Spr

out n

umbe

r per

see

d tu

ber

0

2

4

6

Figure 3.3. Influence of storage duration (weeks) at 4oC on sprout number per seed tuber in Atlantic ( ) and Granola ( ). Vertical bars are l.s.d. values at P = 0.05 within storage duration. The relationship between storage duration and sprout number was linear with equation of y = -3.64 + 0.25x , r2 = 0.99 in Atlantic and y = – 3.07 + 0.25x, r2 = 0.86 in Granola.

63



22 24 26 28 30

Tim

e of

em

erge

nce

(DA

P)

0

5

10

15

20

Figure 3.4. Influence of storage duration (weeks) at 4oC on time of emergence using Atlantic cut ( ), Atlantic whole ( ), Granola cut (o) and Granola whole ( ) seeds. Vertical bars are l.s.d. values at P = 0.05 within storage duration.

Stem number. Storage duration at 4oC influenced the number of stems developed on the

plants (Figure 3.5). Atlantic whole seeds produced 3.6 stems after being stored for 22

weeks. Stem number remained constant as durations in coolstore were prolonged but at

week 30 stem number decreased (from 3.6 stems in week 22 to 2.4 in week 30).

Granola whole seeds produced 3 stems by week 22 and this increased to an average of

4.6 with all storage duration. Stem number of Atlantic and Granola cut seeds was not

influenced by storage duration.

Seeds that were cut produced more stems per seed potato compared to whole seeds (i.e.

when stem number for the 4 cut seed pieces were combined (Figure 3.6). The number of

stems developed on Atlantic cut seed was about twice that of whole seed for all storage

durations, whilst stem number in cut seed of Granola was 2 to 3 fold higher.

64



22 24 26 28 30

Ste

m n

umbe

r per

pla

nt

0

2

4

6

8

Figure 3. 5. Influence of storage duration (weeks) at 4oC on stem number per plant using Atlantic cut ( ), Atlantic whole ( ), Granola cut (o) and Granola whole ( ) seeds. Vertical bars are l.s.d. values at P = 0.05 within storage duration.


22 24 26 28 30

Ste

m n

umbe

r per

pla

nt

0

2

4

6

8

10

12

14

16

18

Figure 3.6. Influence of storage duration (weeks) at 4oC on stem number per plant where four pieces of the same origin were combined using Atlantic cut ( ), Atlantic whole ( ), Granola cut (Ο) and Granola whole ( ) seeds. Vertical bars are l.s.d. values at P = 0.05 within storage duration

65


Plant height. Unlike the other parameters plant height generally decreased in both

Atlantic and Granola (Figure 3.7). Plant height of Atlantic and Granola cut seeds was

about 17 cm after initial storage and then decreased to about 11 cm after 24 weeks and

this remained constant up to week 28. Plant height dropped to 8 cm after 30 weeks

storage. Plant height in whole Atlantic seeds was 11 cm after 22 weeks storage and then

decreased to 7 cm after 24 weeks storage and remained constant for the rest of the

storage duration. Plant height in Granola whole seeds fluctuated with prolonged time in

cool storage. It is suggested that plant height was simply a reflection of duration of

growth.


22 24 26 28 30

Pla

nt h

eigh

t (cm

)

0

5

10

15

20

Figure 3.7. Influence of storage duration (weeks) at 4oC on plant height 30 days after planting using Atlantic cut ( ), Atlantic whole ( ), Granola cut (o) and Granola whole ( ) seeds. Vertical bars are l.s.d. values at P = 0.05 within storage duration.

Field experiments

Emergence. The untreated tubers in experiment 2 produced their first shoot 15 DAP

(Atlantic) and 19 DAP (Granola). Complete shoot emergence occurred 20 DAP in

Atlantic and 24 DAP in Granola. The application of paraquat + diquat did not influence

shoot emergence because it was applied after shoot emergence. Apical sprout removal

did not influence first emergence but it hastened the completion of shoot emergence in

Granola by one day (Table 3.2).

66


Carvone did not influence the time that the first shoot emerged or when shoot

emergence was complete in both Atlantic and Granola (Table 3.3).

Table 3.2. Influence of apical shoot removal (ASR) on time (days after planting, DAP) of first and complete emergence (DAP) in Experiment 2. Means within columns and varieties followed by the same letters are not significantly different at P = 0.05.

Variety Treatment First emergence Complete emergence (DAP) (DAP) Atlantic Control 14.7a 19.7a ASR 15.2a 20.0a Granola Control 18.5a 24.0b ASR 18.5a 23.0a l.s.d. (P = 0.05) 0.9 0.9

Table 3.3. Influence of carvone on time (days after planting, DAP) of first and complete emergence in Experiment 3 Means within columns and varieties followed by the same letters are not significantly different at P = 0.05.

Variety Carvone First emergence Complete emergence (mg/kg) (DAP) (DAP) Atlantic 0 14.2a 19.7a 0.6 15.2a 20.2a Granola 0 18.2a 25.7a 0.6 18.5a 26.7a l.s.d. (P = 0.05) 1.3 2.0

Shoot dry weight (DW). In Experiment 2, neither paraquat + diquat nor apical sprout

removal influenced the shoot dry weight, which was 43 g in Atlantic and 53 g in

Granola (Table 3.4).

Table 3.4. Influence of rate and timing (early and late tuber initiation) of paraquat + diquat application on shoot dry weight (g) 82 DAP in Experiment 2.

67


Means within columns and varieties followed by the same letters are not significantly different at P = 0.05.

Variety Paraquat + diquat Time of application (mL/ha) Early tuber initiation Late tuber initiation Atlantic 0 42.7a 42.7a 250 38.1a 40.7a 500 34.8a 42.2a Granola 0 53.5a 53.5a 250 45.5a 57.8a 500 51.5a 42.2a l.s.d. (P = 0.05) Variety x rate x timing 21.1

Chlorophyll content. In Experiment 2, leaf chlorophyll content decreased with increased

rates of paraquat + diquat applied at both early and late tuber initiation (Table 3.5).

Application of paraquat + diquat at early tuber initiation decreased chlorophyll content

in Atlantic from 7.7 to 5.3 mg/g DW when paraquat + diquat was applied at a very low

rate (250 mL/ha) whilst in Granola the chlorophyll content decreased from 8.9 to 4.8

mg/g DW when a very low rate of paraquat + diquat was applied and was further

decreased to 4.6 mg/g DW with low rate. Application of paraquat + diquat at late tuber

initiation at very low rate did not influence chlorophyll content in Atlantic but

chlorophyll content decreased from 7.7 to 4.8 mg/g DW with low rate whilst in Granola

chlorophyll content decreased from 8.9 to 5.5 mg/g DW with very low rate and to 4.8

mg/g DW with low rate.

Table 3.5. Influence of rate and timing (early and late tuber initiation) of paraquat + diquat application on leaf chlorophyll (mg/g DW) content 84 DAP in Experiment 2.

68



Variety Paraquat + diquat Time of application (mL/ha) Early tuber initiationLate tuber initiation Atlantic 0 7.7b 7.7b 250 5.3a 7.6b 500 6.0a 4.8a Granola 0 8.9b 8.9b 250 4.8a 5.5a 500 4.6a 4.8a l.s.d (P = 0.05) Variety x rate x timing 1.4

Stem number per plant. In Experiment 2, the number of stems developed on Atlantic

was 3 and in Granola was 2.6 and was not influenced by paraquat + diquat application.

Apical sprout removal did not influence the number of stems developed by Atlantic but

it increased from 2.6 to 3.3 in Granola (Table 3.6). In Experiment 3, carvone did not

influence stem number in both Atlantic and Granola (Table 3.6).

Table 3.6. Influence of apical sprout removal (ASR) on stem number per plant 82 DAP in Experiment 2 and carvone 74 DAP in Experiment 3. Means within rows followed by the same letters are not significantly different at P = 0.05.

Variety Apical sprout Carvone (mL/kg) intact removed 0 0.6 Atlantic 3.0a 3.1a 3.2a 3.3a Granola 2.6a 3.3b 2.8a 2.6a l.s.d. P = 0.05 0.6 0.4

Tuber number per plant at final harvest. In Experiment 2, paraquat + diquat applied at

early or late tuber initiation did not influence tuber number or tuber size distribution.

The exception was greater development of tubers 20-55 g in Granola, increasing from

1.3 to 1.9 tubers (Table 3.7).

Table 3.7. Influence of rates and timing (early and late tuber initiation) of paraquat + diquat application on tuber number in different size grades (g) and total tuber number at final harvest 146 DAP in Experiment 2.

69



Variety Paraquat + diquat Tuber number per plant in different size grades (g) mL/ha 20-55 56-110 111-200 201-250 >250 Total Paraquat + diquat applied at early tuber initiation Atlantic 0 0.6a 1.4a 1.9a 0.9a 1.0b 5.8a 250 0.7a 1.6a 2.3a 1.0a 0.7ab 6.4a 500 0.7a 1.4a 2.0a 0.8a 0.6a 5.5a Granola 0 1.3a 3.0a 2.7a 0.4a 0.3a 7.8a 250 1.8b 3.0a 2.3a 0.4a 0.1a 7.8a 500 1.9b 2.5a 2.1a 0.5a 0.0a 7.0a Paraquat + diquat applied at late tuber initiation Atlantic 0 0.6a 1.4a 1.9a 0.9a 1.0a 5.8a 250 0.6a 1.3a 2.3a 1.0a 1.0a 6.3a 500 0.5a 1.4a 2.0a 0.9a 0.9a 5.7a Granola 0 1.3a 3.0a 2.7a 0.4a 0.3a 7.8a 250 1.3a 3.0a 2.8a 0.6a 0.2a 8.0a 500 1.0a 3.3a 2.9a 0.4a 0.3a 8.0a l.s.d. (P = 0.05) Variety x rate x timing 0.4 0.9 0.6 0.4 0.3 1.3

Apical sprout removal influenced tuber number in some of the tuber size grades and in

Granola it increased total tuber number (Table 3.8). Apical sprout removal did not

influence tuber number of 20-55 g in Atlantic but in Granola the number of tuber

increased from 1.3 to 1.8. The number of tubers 56-110 g was greater for both varieties

when apical sprout were removed. The number of tubers 111-200 g in Granola also

increased from 2.7 to 3.2. The total number of tubers developed by Atlantic (5.9 on

average) was not affected by apical sprout removal but it increased in Granola from 7.8

to 9.7.

Table 3.8. Influence of apical shoot removal (ASR) on tuber number per plant in different size grades and total tuber number at final harvest 146 DAP in Experiment 2. Means within columns and varieties followed by the same letters are not significantly

70


different at P = 0.05.

Variety Treatment Tuber number per plant in different size grades (g) 20-55 56-110 111-200 201-250 >250 Total Atlantic Control 0.6a 1.4a 1.9a 0.9a 1.0a 5.8a ASR 0.5a 3.0b 1.9a 1.1a 0.9a 6.0a Granola Control 1.3a 1.5a 2.7a 0.4a 0.3a 7.8a ASR 1.8b 3.6b 3.2b 0.6a 0.3a 9.7b l.s.d. (P = 0.05) 0.3 0.5 0.3 0.2 0.2 0.7

In Experiment 3, carvone application influenced tuber number per plant for some tuber

size categories but did not significantly alter total tuber number (Table 3.9). Tuber

number in the size class 111-200 g Atlantic increased from 2.2 to 2.9 and in Granola

from 2.7 to 3.4. The number of large tubers (201-250 and >250 g) decreased. Total

tuber number was not influenced by carvone, which were 6.8 in Atlantic and 9.4 in

Granola.

Table 3.9. Influence of carvone on tuber number per plant in different size grades and total tuber number at final harvest 147 DAP in Experiment 3. Means within columns and varieties followed by the same letters are not significantly different at P = 0.05.

Variety Carvone Tuber number per plant in different size grades (g) (mL/kg) 20-55 56-110 111-200 201-250 >250 Total Atlantic 0 1.0a 1.8a 2.2a 1.0b 0.7b 6.8a 0.6 0.9a 1.4a 2.9b 0.5a 0.3a 6.8a Granola 0 2.0a 3.4a 2.7a 1.0b 0.6b 9.3a 0.6 1.6a 3.3a 3.4b 0.7a 0.3a 9.6a l.s.d. (P = 0.05) 0.4 0.4 0.4 0.2 0.2 0.7

Yield at final harvest. In Experiment 2, paraquat + diquat applied at early tuber

initiation at a low rate reduced yield of the largest tuber size class (>250 g) from 24.2 to

15.5 t/ha and total yield decreased from 77 to 67 t/ha. In Granola, yield of large tubers

(>250 g) decreased from 8.6 to 2.7 t/ha when a very low rate of paraquat + diquat had

been applied and was further decreased to 1.2 t/ha when a low rate of paraquat + diquat

had been applied. Total yield decreased from 72 to 62 t/ha. For both varieties, paraquat

71


+ diquat applied during late tuber initiation did not influence tuber yield amongst the

different size classes or the total yield (Table 3.10).

Table 3.10. Influence of rates and timing (early and late tuber initiation) of paraquat + diquat application on yield (t/ha) in different size grades (g) and total yield at final harvest 146 DAP in Experiment 2. Means within columns and varieties followed by the same letters are not significantly different at P = 0.05.

Variety Paraquat + diquat Yields (t/ha) in different size grades (g) (mL/ha) 20-55 56-110 111-200 201-250 >250 Total Paraquat + diquat applied at early tuber initiation Atlantic 0 2.2a 10.2a 22.9a 17.1a 24.2a 76.6b 250 2.5a 11.5a 29.3a 18.0a 18.5a 79.7b 500 2.5a 9.7a 25.8a 13.9a 15.5b 67.4a Granola 0 4.7a 19.9a 30.8a 7.7a 8.6a 71.8b 250 6.1a 19.4a 26.4a 7.6a 2.7a 62.3a 500 6.1a 22.5a 23.8a 8.2a 1.2a 61.8a Paraquat + diquat applied at late tuber initiation Atlantic 0 2.1a 10.2a 22.9a 17.1a 24.2a 76.6a 250 2.1a 9.6a 28.8a 17.3a 27.6a 85.4a 500 1.6a 10.0a 26.8a 15.9a 23.0a 77.3a Granola 0 4.7a 19.9a 30.8a 7.7a 8.6a 71.8a 250 4.4a 20.6a 32.1a 10.2a 4.6a 71.9a 500 3.6a 21.5a 32.8a 6.8a 7.8a 72.5a l.s.d. (P = 0.05) Variety x rate x time 1.7 3.8 7.5 7.1 7.4 9.1

In Granola but not Atlantic, apical sprout removal significantly increased both the yields

of the various size grades of tubers and the total yield (Table 3.11). In Granola, the yield

of 20-55 g tubers increased by 32% and of 56-110 g potatoes increased by 21% and

111-200 g size grade by 7%. Yield of 201-250 increased by 14%, yield of the largest

grade was not affected and total yield increased by 18%.

Table 3.11. Influence of apical shoot removal (ASR) on tuber yield (t/ha) in different size grades and total yield at final harvest 146 DAP in Experiment 2.

72



Variety Treatment Tuber yield (t/ha) in different size grades (g) 20-55 56-110 111-200 201-250 >250 Total Atlantic Control 2.2a 10.2a 22.8a 17.2a 24.2a 76.6a ASR 1.8a 10.7a 24.1a 19.6a 25.3a 81.6a Granola Control 4.7a 19.9a 30.8a 7.7a 8.6a 71.8a ASR 6.2b 24.1b 36.1b 11.0b 6.8a 84.4b l.s.d. (P = 0.05) 0.9 2.2 4.4 4.3 4.3 5.5

The application of carvone influenced the yield of tubers in the different size classes

according to variety (Table 3.12). In Atlantic, the yield of 56-110 g potatoes decreased

by 30% but that of the other size classes was not influenced. Total tuber yield in

Atlantic was decreased by 12%. In Granola, the yield of 20-55 g potatoes decreased by

27% with carvone application whilst the yield of other size classes and the total yield

were not influenced.

Table 3.12. Influence of carvone on yield (t/ha) in different size grades (g) and total yield at final harvest 147 DAP in Experiment 3. Means within columns and varieties followed by the same letters are not significantly different at P = 0.05.

Variety Carvone Tuber yield (t/ha) in different size grades (g) (mL/kg) 20-55 56-110 111-200 201-250 >250 Total Atlantic 0 3.7a 13.9b 27.9a 18.9a 17.8a 82.3b 0.6 3.7a 10.6a 24.9a 18.9a 15.3a 73.5a Granola 0 7.5b 23.8a 34.6a 9.8a 6.5a 82.2a 0.6 5.9a 22.5a 39.9a 11.9a 6.6a 86.9a l.s.d. (P = 0.05) 1.5 2.7 6 2.9 5.5 6.2

3.4. Discussion

Effects of storage duration at 4oC

Prolonged storage duration of seed tubers at 4oC increased sprout number per seed in

both Atlantic and Granola. This has been observed with other varieties (Hartmans and

Van Loon 1987) and it is generally agreed that this is related to a reduction of apical

73


dominance in tubers with advancing age during storage (Knowles and Bottar 1991;

Kumar and Knowles 1993).

Extending the time in cool store for both Atlantic and Granola also reduced the time

taken for complete emergence of all shoots. This is not surprising considering that

shorter time intervals for complete shoot emergence has been reported for similar

treatments with other varieties (Iritani 1968). Indeed, apical dominance is known to be

reduced with increasing tuber age resulting from prolonged storage (at 4oC)

(Bodlaender and Marinus 1987; Mikitzel and Knowles 1989a; Mikitzel and Knowles

1990b).

The loss of apical dominance in aging tubers is correlated with a reduction in auxin

biosynthesis and reduced basipetal auxin transport. Furthermore, older seed tubers are

known to have a greater capacity to catabolise auxin (Kumar and Knowles 1993). The

production of more sprouts and stems after prolonged storage of Atlantic and Granola

tubers at 4oC may also be related to the release of lateral buds from correlative

inhibition through decreased endogenous axuin levels, either by reduced auxin transport

or increased auxin catabolism in ageing tubers (Kumar and Knowles 1993).

Stem number in Atlantic whole seeds did not influence by storage duration except

duration of 30 weeks decreased stem number and this probably indicating excess aging

(Kawakami, 1962). Stem number in Granola whole seeds increased indicating lost of

apical dominance with prolonging time in cool storage.

In the present experiments, cut seeds produced more stems per unit weight of seeds than

whole seeds, suggesting that apical dominance had somehow been altered. In the cut

seed of both varieties there was a greater potential of lateral buds to initiate stems. This

may be related to a reduction of apical dominance mediated by endogenous gibberellin

synthesis induced by the wounded tissues (Rappaport and Lippert 1967). This wound-

induced GA synthesis would also promote sprout growth and elongation. Furthermore,

74


some apical sprouts would have been damaged by cutting sprouted seeds and thus

removed some of the primary sites of auxin synthesis. Under these conditions the

correlative inhibition of lateral buds by the apical buds would be decreased resulting in

outgrowth of lateral buds, producing more stems (Michener 1942; Woolley and

Wareing 1972b; Knowles et al. 1985; Kumar and Knowles 1993).

Effects of apical sprout removal

Removing the longest apical sprout at the rose end of the tuber increased the number of

stems developed by Granola but not Atlantic. In Granola, this treatment probably

removed the source of correlative inhibition (i.e. auxin produced in apical buds)

enhancing lateral sprout growth (Michener 1942; Knowles et al. 1985; Kumar and

Knowles 1993). However, removing the longest apical sprout did not influence the

number of stems produced by Atlantic. This suggests that Atlantic may be more

strongly apically dominant than Granola and that removing only the longest sprout from

the rose end was not sufficient to reduce apical dominance in this variety. In addition to

the longest apical sprout at the rose end of Atlantic tubers there were generally two

other smaller sprouts and the apex of these may have taken over the dominant role thus

inhibiting the growth of lateral sprouts. The smaller sprouts would inhibit the outgrowth

of lateral sprouts probably via continuous auxin production by their apices and hence

auxin transport to the lateral sprouts inhibited their growth. The plausibility of this was

illustrated by Harrington (2000) who found that removal of all the sprouts at the rose

end of Atlantic tubers reduced apical dominance and generated tuber seed with multiple

stems. Alternative reasons that apical sprout removal had no influence on tuber number

in Atlantic might also be related to physiological age of Atlantic seed tubers. In the

present experiment seed had been stored at 4oC for 6 months, then, placed at 20 ± 5oC

for 2 weeks to allow sprouting (2-4 mm) to occur. These Atlantic seeds were probably

75


younger than Atlantic used by Harrington (2000). Although no data about tuber seed

storage history was presented in that study it was stated that apical sprouts were

approximately 5 cm, suggesting that Harrington used much older Atlantic seed tubers

for apical sprout removal.

In the variety Atlantic total tuber number per plant, and yield, were not influenced by

apical sprout removal. Harrington (2000) removing all sprouts from the rose end of

Atlantic increased the number of tubers developed as a result of increased numbers of

stems. In my investigation with Granola, apical sprout removal increased the total

number of tubers developed, increased the number of small and medium size tubers and

increased total yield. This was due to the loss of apical dominance as indicated by the

development of a greater number of stems, which in turn increased the numbers of

tubers developed and total yield. The contrasting findings between Atlantic and Granola

in terms of stem and tuber number and yield in response to apical sprout removal

strongly suggest that they significantly differ in their degree of apical dominance.

Effects of carvone

Carvone application has been known to increase the number of stems produced by

potato seed (Brown et al. 2000) however this was not the case in the present

investigation with Atlantic and Granola where carvone did not influence stem number.

This was probably due to lower headspace concentration of carvone and this relates to

the method of application. Carvone was applied by simmering it with water in a fry pan

and potatoes were placed in a wire rack closed with waterproff tarpaulin. This method

probably could not create optimum and constant headspace carvone concentration like it

does using a fogging applicator as in other experiments, such as that conducted by

Brown (2000). Alternatively the time of carvone application might not be appropriate.

76


The size of sprouts when carvone should be applied is not known and it needs further

investigation.

Compared with controls, there was no increase in the number of stems after carvone

application and thus the number of tubers developed was the same.

Effects of paraquat + diquat

The application of paraquat + diquat reduced total yield and it did not influence the total

number of tubers developed per plant. The paraquat + diquat mixture inhibits the

photosynthetic reactions in photosystem I which is one of the light reactions needed to

form ATP (adenosine triphosphate) and NADPH (nicotinamide adenine dinucleotide

phosphate). These compounds are important for sugar synthesis (Summers 1980). The

reduction in leaf chlorophyll content and probable reduced photosynthetic rates on

plants treated with paraquat + diquat probably reduced the assimilate available for tuber

growth which resulted in their low yield. Reduction in yield occurred at early

application in both Atlantic and Granola. Depletion of assimilates supplies during early

tuber initiation probably reduced starch deposition, which is considered to be one of the

key anatomical changes during early tuber initiation (Duncan and Ewing 1984). Leaves

turned yellow soon after application of paraquat + diquat and 30 days were required for

plants to recover. Tuber bulking commenced during this period. Reduction of

assimilates inhibited tuber bulking thus resulting in lower yield.

Shoot dry weight was not influenced by the application of paraquat + diquat but shoots

had largely established before early tuberization. New shoots weigh very little and their

damage was not detected by a reduction in dry weight.

Paraquat + diquat did not influence total tuber number. If GA synthesis in shoot apices

(Menzel 1981) and transport to stolons (Menzel 1983) was reduced by damaging the

site of synthesis by herbicides it would promote tuber initiation (Krauss 1981; Krauss

77


and Marschner 1982; Koda and Okazawa 1983). However, total tuber number did not

increase, indicating that herbicide application did not influence hormonal regulation of

tuber initiation.

3.5. Conclusions

Prolonging seed tuber storage period at 4oC reduced apical dominance as indicated by a

number of changes including the development of a greater number of sprouts, earlier

sprout emergence and a greater number of stems. Different storage duration was

required by each variety in order to reach the developmental stage associated with the

development of optimal numbers. Since stem number is a very important parameter for

determining tuber number, storage duration should be adjusted to maximize stem

number. Storage duration for 22, 24, 26 and 28 weeks did not influence stem number of

Atlantic whole seeds but it decreased at week 30. Storing whole Granola seeds for 24,

26, 28 and 30 weeks gave the same number of stems and these were higher compared to

storage for 22 weeks.

Removing apical sprouts reduced apical dominance in Granola and this increased the

number of stems and tubers developed and increased total yield and the yield of small

round seeds. In Atlantic however, apical sprout removal did not influence these

parameters. Removing only the longest sprout was probably not sufficient to break

apical dominance over the other, smaller rose-end sprouts. The contradictory results

strongly indicated that Atlantic and Granola have different degrees of apical dominance.

Compared with the control plants, carvone application did not influence the number of

stems and tubers developed and the yield was the same. This was probably due to low

and inconsistent headspace concentration created by a simple method of carvone

application. The time of carvone application may also have been inappropriate.

78


3.6. Recommendations

Atlantic whole seeds should not be cool stored for longer than 28 weeks because after

this time stem number decreased. Granola whole seeds should be cool stored for longer

than 22 weeks (from 24 to 30 weeks) for optimum stem number. The physical removal

of apical sprouts was found to increase small tuber production in Granola but it is not a

practical method for commercial seed potato production. The use of carvone requires

further investigation with different times of application (e.g. different size of sprouts in

seed tubers). The headspace of carvone concentration should be measured and it should

be optimum and constant. The application of paraquat + diquat is not recommended for

use in increasing tuber number, tuber yield or yield of small round seeds potatoes.

79

Chapter 4 GA3 and paclobutrazol for seed potato production

Chapter 4

The use of gibberellic acid and paclobutrazol to increase the yield of

small round seed potatoes (Solanum tuberosum L.) varieties Atlantic

and Granola

4.1. Introduction

Small round seed potatoes of the varieties Atlantic and Granola are in great demand for

export to Indonesia as seed potatoes. Therefore, there is some interest in the

development of techniques that increase yields of small tubers (20-55 g) of these

varieties. There are et least four approaches to increase the yield of small tubers

without reducing total yield. Firstly, increasing stem number by applying physical or

chemical treatments such as plant growth regulators (Sekhon and Singh 1984).

Secondly, improving translocation of assimilates from shoots to the developing tubers

(Kumar et al. 1980; Sekhon and Singh 1985) and thirdly by combining these treatments

(Kumar and Warieng 1974). The fourth approach is to select varieties that naturally

produce only small tubers, but this is constrained by the demand for large tubers in the

fresh and processing markets.

A high number of stems per unit area is important for production of small seed tubers

(Struik and Wiersema 1999). Application of GA breaks dormancy in potatoes

(Claassens and Vreugdenhil 2000; Fernie and Willmitzer 2001) and reduces apical

dominance (Bishop and Timm 1968; Holmes et al. 1970). These factors can stimulate

multiple sprout emergence leading to an increased number of stems that produce a

greater proportion of small tubers (Holmes et al. 1970; Marinus and Bodlaender 1978;

Mikitzel 1990). The use of GA to increase the yield of small round seed in Atlantic and

Granola under field conditions in Western Australia has not been investigated.

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The response to applied GA depends on the variety, concentration of GA, physiological

age of tubers when GA is applied and the method of application. In addition, the

influence of GA will also vary with environmental factors during tuber production and

tuber storage. The most commonly used GA is GA3 and to be effective each variety

requires different concentrations (Holmes et al. 1970; Mikitzel 1993). The range of

concentrations used varies from 0.1 to 100 mg/L and high concentrations may cause

abnormalities (Sekhon and Singh 1984; Mikitzel 1993). The application of GA is not

effective in increasing stem numbers under all conditions with all varieties (Menzel

1980; Sharma et al. 1998b). For each variety, it is important to determine the effective

concentration of GA required and for Atlantic and Granola this can be accomplished

using a range of GA concentrations. These two varieties contrast in growth habit with

Atlantic producing fewer, larger tubers and Granola more, smaller tubers.

Gibberellic acid can be applied with various methods. Dipping seed pieces or whole

seeds in a solution containing GA before planting (Bodlaender and van de Waart 1989;

Mikitzel 1993; van Ittersum and Scholte 1993) or spraying foliage during plant growth

(Caldiz et al. 1998; Sharma et al. 1998b) or applying GA in soil during plant growth

(Struik et al. 1989b).

Paclobutrazol is a triazole, which blocks gibberellin biosynthesis in it oxidative

reactions from ent-kaurene to ent-kaurenoic acid (Rademacher 1999). Growth retardants

are also called anti-gibberellins because they usually have the opposite effect to GA

(Langille and Hepler 1992). Growth retardants applied to plants, retard shoot growth,

hasten tuber initiation and increase tuber number and yield (Dyson 1965; Barry et al.

1990). Unlike shoot growth, tuber initiation is associated with declining endogenous

gibberellin levels (Koda and Okazawa 1983; Xu et al. 1998b) and application of

paclobutrazol should reduce endogenous GA levels (Rademacher 1999).

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Other growth retardants, such as CCC (2-chloroethyltrimethyl ammonium chloride),

increase tuber yield by enhancing the flow of assimilates from shoots to tubers (Sharma

et al. 1998b) whereas GA directs assimilates away from tubers towards stolons and

shoots (Sharma et al. 1998b).

Paclobutrazol increases tuber number and yield of potatoes grown in pots (Balamani

and Poovaiah 1985), increases production of mini-tubers (Bandara and Tanino 1995;

Bandara et al. 1998) and increases the proportion of explants that develop tubers in

tissue culture (Simko 1993; Simko 1994). However, information about plant growth and

the response of tuberization to paclobutrazol on potatoes under field conditions is

lacking, especially with Atlantic and Granola. The present study investigated the

influence of paclobutrazol application rate (at early tuber initiation) on seed potato

production.

A combination of GA (as a pre-soak treatment) and paclobutrazol as a foliar spray (after

shoot emergence) has been very successful for the cultivation of flower crops such as

Zantedeschia in pot culture (Corr and Widmer 1991). The application of GA increases

flower number and paclobutrazol reduces plant height. The influence of this chemical

combination on tuber growth for potatoes grown under field conditions has not been

tested before and it may have a similar, beneficial response.

In controlled experiments the combined treatment of GA3 and CCC increased tuber

number (Kumar and Warieng 1974) but in other experiments this treatment did not

influence the number of tubers (Dyson 1965). Paclobutrazol is more potent than CCC

(Rademacher 1999) therefore the combination of GA3 and paclobutrazol may promote

tuberization. Application of gibberellic acid to Atlantic and Granola seed pieces before

planting may promote multiple stem growth, increase the numbers of tubers and

increase competition between tubers. If paclobutrazol were applied following GA

application during early tuber initiation this may de-suppress the inhibition of

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tuberization, retard shoot and stolon growth and increase the flow of assimilates from

shoots to tubers. Not only could the combination of chemical treatments provide

increased numbers of tubers but also increase the proportion of small round seed tubers

developed without reducing total yield. The results may have important implications for

the development of practical methods aimed at increasing commercial production of

small round seed potatoes in Australia. Specifically, the aim of these experiments was

to increase yield of small round seeds weighing 20-55 g without reducing total yield.

The hypotheses tested were i) GA application increases stem and tuber number, ii) GA

application increases the proportion of small size tubers developed without reducing

total yield, iii) Paclobutrazol promotes tuberization and iv) Gibberellic acid and

paclobutrazol in combination provide a synergistic effect to increase the yield of small

seeds without reducing total yield.

4.2. Materials and Methods

Sites characteristics

Two field experiments were conducted. The first experiment was at the Department of

Agriculture Western Australia, Manjimup Horticultural Research Institute (34o18’S,

116o 7’E) in a sandy loam soil from October 2001 to March 2002. The second

experiment was conducted at the University of Western Australia, Shenton Park

Research Station, Perth (31o56’S; 115o47’E) in sandy soil from August to December

2002. Chemical and physical characteristics of the top 15 cm of soils are presented in

Table 4.1 and the weather conditions in Table 4.2.

Table 4.1. Chemical and physical characteristics of top 15 cm of soil collected before experiments began for Experiment 1 and 2.

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Parameter Unit Site 1 (Manjimup) Site 2 (Perth) pH (CaCl2) - 5.5 6.5 P mg/kg 135.0 45.0 K mg/kg 79.0 40.3 NO3-N mg/kg 13.0 2.0 NH4-N mg/kg 4.0 1.0 Total N % 0.13 - Organic C % 2.7 0.3 Texture - Sandy loam Sand Colour - Brown Light brown

P and K were measured as per Colwell (1963), NO3-N as per Reardon et al., (1966), NH4 as per Best (1976)

Table 4.2. Weather data during experiments. Values are means of monthly data except rainfall, which is the total for each month. Day lengths were calculated by subtracting sunrise from sunset times as published online by Perth Observatory.

Month Minimum Maximum Day length Relative Total rain

temperature

(oC) temperature

(oC) (hours) humidity (%) (mm) Exp 1 (Manjimup) October (2001) 7.0 16.2 12.9 87.7 40.8 November 9.9 21.0 13.8 80.6 36.8 December 9.4 20.2 14.4 82.6 81.4 January (2002) 11.6 25.1 14.0 77.0 4.6 February 12.4 25.3 13.6 72.4 5.8 March 13.0 26.1 11.6 76.1 10.0 Mean 10.5 23.3 13.4 79.4 29.9 Exp 2 (Perth) August (2002) 7.6 18.6 10.9 74.4 81.2 September 9.0 19.8 11.9 70.4 85.6 October 11.5 22.3 12.9 67.2 42.2 November 13.7 26.6 13.5 58.3 25.8 December 17.4 30.4 14.1 50.7 3.8 Mean 11.8 23.5 12.7 64.2 47.7

Crop management

Certified seeds of Solanum tuberosum cv. Atlantic, first generation (G1), and cv.

Granola, second generation (G2) were used. These seeds were cool stored for 24 weeks

from harvest to planting. Seeds were transferred from cool storage for sprouting 2

weeks before planting and held at 20oC. Potatoes were mechanically planted on 25

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October 2001 in Experiment 1 and on 7 August 2002 in Experiment 2 at 15 cm depth

with an in-row spacing of 0.15 m (83,333 plants/ha) with 0.8 m between row centers.

In Experiment 1 irrigation was applied using impact sprinklers at 220 kPa and

monitored with tensiometer. Eight tensiometers were installed in the hills of main rows

with 2 tensiometers depth at 30 and 50 cm in each main row soon after shoots has

emerged. Readings of tensiometers gauge were taken every 2 days and irrigation was

applied when readings fell to ≤ 20 cb at 30 cm depth.

In Experiment 2 irrigation was applied using impact sprinklers at 220 kPa.

Tensiometers were installed on control sub plot of each variety at 15, 30 and 45 cm

depth. Readings were taken as above and irrigation was applied when the reading fell to

≤ 5 cb at 15 cm depth.

Fertilizer was applied at different rates and times because the sites had different soil

properties. In Experiment 1, potassium (82 kg K/ha as K2SO4) was applied 10 days

before planting and the following fertilizers were banded at the time of sowing. A trace

element plus magnesium mix containing (kg/ha) Mg (13), Zn (7), Mn (7), S (7), Fe (2),

Cu (2), B (0.6) and 56 g Mo/ha was applied. Total Nitrogen (N), Phosphorus (P) and

Potassium (K) applied were 200, 250 and 311 kg/ha respectively. Half the N was

applied at sowing and the other half at 100 % crop emergence.

In Experiment 2, Potassium (82 kg K/ha), Phosphorus (200 kg P/ha) and a trace element

mix plus magnesium containing (kg/ha) Fe (3.6), Mg (5), Mn (6.5), Zn (5.8), B (2), Cu

(4.5), and Mo (0.9) were applied before planting. Nitrogen (50 kg N/ha) was applied

after planting but before shoots had emerged. After emergence, N (32 kg/ha) and K (40

kg/ha) were applied on a weekly basis and Mg (8 kg/ha) once every three weeks for 12

weeks. In total, crops received 434 kg N/ha, 200 kg P/ha and 562 kg K/ha. Crops were

monitored for insects, pests and diseases and treated as necessary. In Experiment 1, the

insecticides methamidophos (700 mL Nitofol®/ha) and permethrin (200 mL

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Ambush®/ha) were applied to protect plants against potato moth and green peach aphid.

Difenoconazole (200 mL/ha Score®) and chlorothalonil (2 L Bravo®/ha) were applied in

rotation on a weekly basis to control target spot (Alternaria solani). Metribuzin (500

mL Sencor® /ha) and paraquat dichloride (3 L Gramoxone® /ha) was applied

immediately after crops and weeds emerged.

In Experiment 2, weeds were hands removed. A bio-insecticide, (425 g Dipel®/ha

containing 4320 IU Bacillus thuringiensis var Kurstaki, Arthur Yates & Co.Limited,

NSW Australia) was applied to protect plans against leaf-eating caterpillars. The

fungicide difenoconazole (500 mg Score®/ha) was applied to control target spot.

In both experiments haulms were killed using paraquat + diquat (3 L Spray.Seed®/ha)

when 50% of plants had senesced. They were harvested mechanically two weeks after

haulms were killed (146 days after planting for Experiment 1) and (118 DAP for

Experiment 2)

Treatments

In Experiment 1, paclobutrazol (Cultar®, contains 250 g paclobutrazol/L as an active

ingredient, Crop Care Australia Pty Ltd, Pinkenba Qld) at 0, 100 and 250 mg /L was

applied as a foliar spray at early tuber initiation when swelling tubers were twice the

stolon diameter (Firman et al. 1991). This was 42 DAP for Atlantic and 46 DAP for

Granola. Control plants were sprayed with water.

In Experiment 2, freshly cut seed pieces (50 ± 5 g) were dipped for 15 minutes in GA3

treatment solution containing 0, 5, 20, 40 mg a.i./L (Grando GA3® contains 100 g

GA3/L as an active ingredient, Aftern Ltd, Perth) prepared with deionised (DI) water

two days before planting. Control seeds were dipped in DI water. Seeds were air-dried

in ambient conditions (20oC) and planted as above. Four concentrations of

paclobutrazol (0, 100, 250 and 350 mg/L) were applied 34 DAP for GA3-treated plants

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and 42 DAP for non-GA3 treated plants to co-inside with early tuber initiation, which

was monitored in the field in buffer rows.

Experimental design

The experimental design was a split plot, with varieties (2) as the main plot randomized

within each block. In Experiment 1, paclobutrazol treatments were sub plots

randomized within main plots. Main plot size was 2.4 m wide by 30 m long and sub

plot size 2.4 m wide by 5 m long with 3 rows. In Experiment 2, a factorial of four

concentrations of GA3 and four concentrations of paclobutrazol were sub plots

randomized within main plots. Main plot size was 1.6 m wide by 64 m long and sub

plot size was 1.6 m wide by 4 m long double rows. Each treatment was replicated four

times.

Plant measurements at sampling

For both Experiment 1 and 2 the time of first and complete plant emergence was

recorded. In Experiment 1, twelve plants per treatment were sample harvested, 74 and

104 DAP from buffer rows. Main stem number (stems arise directly from seed tuber)

per plant, tuber number per plant, stolon + root dry weight (DW), shoot DW and tuber

number per plant were measured. All tubers, which had swelled to twice the stolon

diameter, were counted (O' Brien et al. 1983).

In Experiment 2, 8 plants were harvested per treatment 69 DAP from each end of row to

measure main stem number per plant, length of the longest stolon, stolon + root and

shoot DW, length of internode between the 5 th and the 6 th node from the shoot tip,

plant height and tuber number per plant. All tubers (≥2x stolon diameter) were counted

(O' Brien et al. 1983). Leaf area and leaf chlorophyll content were measured 76 DAP.

Twelve fully expanded leaves were taken from 3 plants per treatment with 4

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replications to measure leaf area using a portable area meter (LI-Cor model LI 3000

USA).

Chlorophyll analysis. Chlorophyll content of leaves in Experiment 2 was measured in

the field using chlorophyll meter (Minolta SPAD 502, Osaka, Japan). Twelve leaves

from 3 plants per treatment were measured. Each leaf was measured (4x) on the left and

right side of a midrib and the average values were recorded. Values of SPAD readings

were calibrated using a destructive method modified from Marquad and Tipton (1987),

Wellburn (1994) and Donelly et al. (2001).

Leaves (50) of various ages and positions in the canopy were harvested and chlorophyll

content was measured with the chlorophyll meter. Leaf discs (981 mm2) were excised

using a cork borer and extracted in 1.5 eppendorf tubes containing 1.5 mL cold absolute

methanol. Tubes were shaken for 24 hours at 4oC until discs became colourless. An

aliquot (1 mL) was transferred to a UV cuvette and absorbance was measured at 653 nm

and 666 nm using a double beam spectrophotometer (Shimadzu UV-1601, Japan).

Extraction and measurement procedures were done in the dark to avoid chlorophyll

degradation. Chlorophyll a and b contents were calculated using equations from

Wellburn (1994):

Chlorophyll a = 15.65 A666-7.34A653

Chlorophyll b = 27.05A653-5.32A666

The chlorophyll meter readings were regressed against chlorophyll a+b content

expressed in mg/m2 leaf area.

Carbohydrate analysis. Tubers of GA3 treated and untreated plants were analyzed for

their carbohydrate content at 69 DAP. Soluble sugar extraction was based on the

methods of Oparka (1985) and Sergeeva et al. (2000) with some modifications.

Preliminary experiments indicated that deionised (DI) water extracted more sugar than

88


ethanol (80% v/v) therefore DI water was used. Freeze dried, ground samples (10 mg)

were placed in 100 mL flasks with water (20 mL) and this was covered with a smaller

flask (50 mL). Flasks were refluxed for 30 minutes in a constantly heated sand bath

(100oC). Material was extracted twice. Control flasks (3) with DI water only were

included in the extraction and quantification procedures as reagent blanks for accurate

analysis. Extracts were filtered using Whatman #1 filter paper and residues were

washed (3x) with warm DI water and re-filtered. All washings and extracts were

combined to make a final volume of 30 mL.

Residues held in filter paper from soluble sugar extractions were used for extraction of

insoluble sugars, such as starch, using HCl (Hassid et al. 1941). Residues in flasks were

left over night in a fume hood to evaporate the water. Hydrochloric acid (40 mL, 3%

v/v) was added to flasks and heated at 100oC for 3 hours to hydrolyze starch to glucose.

Anti bumping granules were added to avoid vigorous boiling and splashing. Triplicate

control samples of pure potato starch (10 mg each) were included in the digestion and

quantification for recovery. Extracts were cooled at room temperature and centrifuged

at 5000 rpm for 5 minutes. Supernatants were collected in 50 mL vials and DI water

was added to make a final volume of 50 mL for analysis.

Total sugar and starch were quantified calorimetrically using anthrone reagent (False

1951). The reagent was prepared in a fume hood by dissolving 400 mg of anthrone

(C14H10O, Merck Schuchrdt, Germany) in concentrated sulphuric acid (200 mL) to

produce reagent A. For reagent B, 15 mL ethanol (95% v/v) and 60 mL DI water was

mixed and chilled on ice. Reagent C (anthrone reagent) was made by mixing reagent A

and reagent B slowly while keeping the latter stirred in an ice bath as the reaction is

exothermic. The anthrone reagent is bright yellow and was cooled to 2-4oC before use.

Sample extracts and standards (0.5 mL) were mixed with anthrone reagent (5 mL) in a

30 mL test tube. Mixtures were boiled for 10 minutes and immediately cooled in an ice

89


bath, vortex mixed and equilibrated at room temperature. The solution (1 mL) was

transferred into a cuvette and absorbance measured at 620 nm using Shimadzu UV-

1601 spectrophotometer (Shimadzu Corporation, Kyoto, Japan). Samples were diluted

when absorbance exceeded 2 AU (absorbance unit). Absorbance of standards

containing 0, 10, 25, 50, 100 and 200 mg/L glucose prepared in DI water were plotted

and the linear regression was used to calculate mg/L of glucose in sample extracts.

Measurement at final harvest

Potatoes were mechanically harvested from main rows. In Experiment 1, 5 m of central

rows were harvested 146 DAP. In Experiment 2, 2 m of main rows were harvested 118

DAP. Potatoes were graded (<20, 20-55, 56-110, 111-200, 201-250 and >250 g). The

number of tubers and their total weight were measured in each size grade. Diseased and

machine-damaged tubers were discarded. Tubers were inspected for internal defects

such as hollow heart and black heart but no evidence of these disorders was observed.

Statistical analysis

The data were analyzed using analysis of variance for a split plot design using Genstat

6.1 (Lewes Agricultural Trust, Rothamsted Experimental Station). Differences between

means of treatments were compared using least significant differences at P = 0.05.

4.3. Results

Emergence

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In Experiment 1, the first plant emergence by control plants was 14 DAP for Atlantic

and 18 DAP Granola with complete plant emergence occurring at 20 and 26 DAP

respectively.

In Experiment 2, the first plants emerged at 25 DAP for both Atlantic and Granola

(Table 4.3) and the plants emerged faster at higher concentration of GA3. Compared

with the control plants, first plant emergence in both varieties was accelerated by 4, 6

and 8 days when applied GA concentrations were 5, 20 and 40 mg GA3/L respectively.

Complete plant emergence of GA3 treated Atlantic plants was 3-4 days earlier and

Granola plants 2-3 days earlier than controls.

Table 4.3. Influence of GA3 on first and complete in Experiment 2. Means within columns and varieties followed by the same letter are not significantly different at P = 0.05.

Variety GA3 First emergence Complete emergence (mg/L) (DAP) (DAP) Atlantic 0 24.8d 26.0b 5 20.9c 22.7a 20 19.1b 22.2a 40 17.2a 22.0a Granola 0 24.7d 25.3b 5 21.4c 23.0a 20 19.0b 22.0a 40 17.1a 23.0a l.s.d. (P = 0.05) 1.5 2.1

Starch and total sugar content in tubers

In Experiment 2, starch and total sugar content in developing potato tubers at 69 DAP

were not influenced by GA3 application (Table 4.4). Starch content in Atlantic was 286

mg/g DW and in Granola 215 mg/g DW. Total sugar content in Atlantic was 152 mg/g

DW and in Granola it was 175 mg/g DW.

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Table 4.4. Influence of GA3 on starch and total sugar content (mg/g dry weight) in developing potato tubers 69 days after planting in Experiment 2. Means within columns and varieties followed by the same letter are not significantly different at P = 0.05.

Variety GA3 Starch Total sugar (mg/L) (mg/g dwt) (mg/g dwt) Atlantic 0 286.2a 152.8a 5 245.3a 195.9a 20 191.0a 215.9a 40 219.2a 198.7a Granola 0 215.4a 175.1a 5 231.3a 169.8a 20 209.0a 180.6a 40 274.0a 233.8a l.s.d. (P = 0.05) 85.9 66.2

Stolon length

In Experiment 2, the length of the longest stolon in the control was 23 cm in Atlantic

and 17 cm in Granola (Table 4.5). Stolon length was not influenced by paclobutrazol

application in Experiment 2. Stolon length increased with applied GA3 in both Atlantic

and Granola. Stolon length in Atlantic increased to 37 cm with 20 mg GA3/L but

increasing GA3 concentration to 40 mg/L resulted in a smaller increase (29 cm). Stolon

length in Granola increased to 23-26 cm at 5 and 20 mg/L and 29 cm at 40 mg GA3/L.

Combining GA3 and paclobutrazol generally did not influence stolon length compared

with GA3 alone but stolons were longer than untreated plants. In Atlantic, a

combination of 20 mg GA3/L with 100 mg paclobutrazol/L resulted in a decrease in

stolon length from 37 to 25 cm and with 350 mg paclobutrazol/L from 37 to 22 cm.

Table 4.5. Influence of GA3 and paclobutrazol (PAC) on length of the longest stolon (cm) 69 days after planting in Experiment 2.

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Variety GA3 Paclobutrazol (mg/L) (mg/L) 0 100 250 350 Atlantic 0 22.6 17.9 24.7 19.1 5 26.5 28.0 27.8 28.4 20 36.5 25.1 33.0 22.0 40 28.6 31.4 25.3 28.7 Granola 0 16.8 18.2 20.4 13.7 5 25.7 26.1 23.4 25.6 20 22.0 31.2 26.6 33.0 40 29.0 27.4 27.7 26.2 l.s.d. (P = 0.05) Variety x GA3 = 5.4 Variety x PAC = 5.4 Variety x GA3 x PAC = 10.7

Stolon + root and shoot dry weight

Generally, the dry weight of stolons + roots and shoots were reduced by application of

paclobutrazol. In Experiment 1, paclobutrazol application decreased stolon + root and

shoot dry weight by approximately one quarter in Atlantic 74 DAP but this decrease

was not evident by 104 DAP. In Granola, paclobutrazol application decreased stolon +

root and shoot dry weights at both 74 and 104 DAP (Table 4.6).

Table 4.6. Influence of paclobutrazol on stolon + root and shoot dry weights (g) at different harvest times (days after planting) in Experiment 1. Means within columns and varieties followed by the same letter are not significantly different at P = 0.05.

In Experiment 2, paclobutrazol applied at 350 mg/L reduced shoot DW but not stolon +

root DW in Atlantic. In Granola, paclobutrazol application reduced stolon + root but not

shoot, dry weight (Table 4.7). Stolon + root and shoot dry weight was influenced by

GA3 in Atlantic but not in Granola (Table 4.8). In Atlantic stolon + root dry weight

Variety Paclobutrazol Stolon + root dry weight (g) Shoot dry weight (g) (mg/L) 74 DAP 104 DAP 74 DAP 104 DAP Atlantic 0 4.9b 4.5a 53.0b 29.3a 100 3.5a 4.5a 38.0a 30.9a 250 4.3ab 4.2a 45.0ab 30.0a Granola 0 5.6b 6.2b 58.7b 43.1b

100 3.3a 5.2ab 39.3a 38.4ab 250 3.9a 4.6a 38.3a 31.4a

l.s.d. (P = 0.05) 1.2 1 13.3 9.4

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increased from 1.9 to 2.9 g with GA3 and shoot dry weight increased from 12.9 to 22.5 g

at 20 mg/L but there was no further increase at 40 mg/L GA3. In Granola stolon + root

dry weight remained at 1.8 g. There was no significant interaction for either parameter

and between pacobutrazol and GA3

Table 4.7. Influence of paclobutrazol on stolon + root and shoot dry weights (g) 69 days after planting in Experiment 2. Means within columns and varieties followed by the same letter are not significantly different at P = 0.05.

Variety Paclobutrazol Stolon + root DW Shoot DW (mg/L) (g) (g) Atlantic 0 1.9a 13.0b 100 2.0a 13.3b 250 2.0a 12ab 350 1.7a 9.5a Granola 0 1.8b 10.3a

100 1.6ab 10.2a 250 1.4ab 9.8a 350 1.3a 9.8a

l.s.d. (P = 0.05) 0.4 2.9 Table 4.8. Influence of GA3 on stolon + root and shoot dry weight (g) 69 days after planting in Experiment 2. Means within columns and varieties followed by the same letter are not significantly different at P = 0.05.

Internode length and

plant height

Variety GA3 Stolon + root DW Shoot DW (mg/L) (g) (g) Atlantic 0 1.9a 12.9a 5 2.3a 15.5a 20 2.9b 22.5b 40 2.9b 20.7b Granola 0 1.8a 10.3a 5 1.7a 12.5a 20 1.8a 16.2a 40 1.9a 14.1a l.s.d. (P = 0.05) 0.4 2.9

Plant growth regulators influenced internode length in Experiment 2 (Table 4.9).

Paclobutrazol application reduced internode length in Atlantic from 8 to 5 mm at both

100 and 250 mg paclobutrazol/L. In Granola, internode length was halved by

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paclobutrazol application. Gibberellic acid application increased internode length with

increasing concentration in both Atlantic and Granola. Adding paclobutrazol to GA3-

treated plants reduced internode length in both Atlantic and Granola, compared to that

when GA3 was applied alone.

Table 4.9. Influence of GA3 and paclobutrazol (PAC) on internode length (mm) 69 days after planting in Experiment 2. Internode length was measured between the 5 th and 6 th node from shoot tip.


Plant height increased with GA3 but decreased with paclobutrazol in Experiment 2

(Table 4.10). Plant height increased with increasing GA3 concentration from 15 to 32

cm in Atlantic and from 19 to 38 cm in Granola. Combining GA3 and paclobutrazol

together decreased plant height compared to that of GA3 alone in Atlantic. For example,

GA3 (5 mg/L) increased plant height from 15 to 22 cm but this decreased to 11 cm with

addition of 350 mg paclobutrazol/L. In Granola, addition of paclobutrazol with GA3

generally did not influence plant height compared to that of GA3 alone except that the

combination of 5 mg GA3/L and 350 mg paclobutrazol/L reduced plant height.

Table 4.10. Influence of GA3 and paclobutrazol (PAC) on plant height (cm) 69 days after planting in Experiment 2.

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Variety GA3 Paclobutrazol (mg/L) (mg/L) 0 100 250 350 Atlantic 0 15.0 11.5 12 10.75 5 22.2 15.5 12.7 11.2 20 24.5 19.0 17.0 20.0 40 32.2 32.7 25.5 26.2 Granola 0 18.5 10.7 9.0 9.0 5 18.7 16.5 18.7 12.7 20 29.7 27.5 28 26.0 40 37.5 38.2 34 33.5 l.s.d.(P = 0.05) Variety x GA3 = 2.9 Variety x PAC = 2.9 Variety x GA3 x PAC = 5.9

Leaf area and leaf chlorophyll content

Generally, plant growth regulators did not influence leaf area per leaf. Leaf area of

Atlantic was 51 cm2 and Granola was 41 cm2 per leaf (Table 4.11). There was a positive

linear relationship between SPAD readings and extractable chlorophyll content (Figure

4.1) Chlorophyll content was influenced by application of plant growth regulators

(Table 4.12). Paclobutrazol increased the chlorophyll content in Atlantic at 350 mg/L

and in Granola at all paclobutrazol concentrations. Gibberellic acid application

decreased the chlorophyll content by 101 (22%), 73 (16%) and 129 (28%) mg/m2 leaf

area at 5, 20 and 40 mg/L respectively, in Atlantic whilst that in Granola was not

influenced.

Applying both GA3 and paclobutrazol reduced chlorophyll content in Atlantic and it did

not influence that in Granola.

Table 4.11. Influence of GA3 and paclobutrazol (PAC) on leaf area (cm2) in Experiment 2 Leaf areas were measured from 12 fully expanded leaves taken from 3 plants per treatment with 4 replications 76 days after planting using a portable area meter

96


Variety GA3 Paclobutrazol (mg/L) (mg/L) 0 100 250 350 Atlantic 0 50.8 54.2 48.8 51.0 5 53.3 43.2 55.3 52.7 20 46.3 60.5 50.7 47.9 40 46.8 51.9 44.2 49.3 Granola 0 41.2 33.6 30.9 42.1 5 37.0 37.9 46.8 43.5 20 41.4 40.4 41.7 45.6 40 40.2 39.2 41.7 42.3 l.s.d. (P = 0.05) Variety x GA3 = 4.8 Variety x PAC = 4.8 Variety x GA3 x PAC = 9.7 Table 4.12. Influence of GA3 and paclobutrazol on chlorophyll content of leaf (mg/m2 leaf area) 76 days after planting in Experiment 2


97


y = 10.179x - 70.642r2 = 0.8184

0

100

200

300

400

500

600

700

800

0 20 40 60 8

SPAD-502 reading

Chl

orop

hylls

a +

b c

onte

nt (m

g/m

2 le

af)

0

Figure 4.1. The relationship between SPAD-502 reading and extractable chlorophyll a and b content (mg/m2 of leaf area) 76 DAP in Experiment 2. Stem number

In Experiment 1, untreated Atlantic had 3.2 and untreated Granola had 2.8 stems per

plant and paclobutrazol did not influence the numbers of stems developed (Table 4.13) .

In Experiment 2, the number of stems developed by Atlantic increased from 1.5 to 2.2

at low and medium concentrations of paclobutrazol (100 and 250 mg/L) respectively

but high concentrations (350 mg/L) did not influence stem number. In Granola, the

number of stems was not influenced by paclobutrazol (Table 4.14).

Table 4.13. Influence of paclobutrazol on stem number per plant 104 DAP in Experiment 1 Means within rows and varieties followed by the same letter are not significantly different at P = 0.05 Variety Paclobutrazol (mg/L)

0 100 250 Atlantic 3.2a 3.2a 2.9a Granola 2.8a 2.6a 2.7a l.s.d. (P = 0.05) = 0.8

98


Table 4.14. Influence of paclobutrazol on stem number per plant 69 DAP in Experiment 2 Means within rows and varieties followed by the same letter are not significantly different at P = 0.05

Variety Paclobutrazol (mg/L) 0 100 250 350

Atlantic 1.5a 2.2b 2.2b 1.5a Granola 2.0a 1.9a 2.4a 2.5a l.s.d. (P = 0.05) = 0.6

In Experiment 2, gibberellic acid application increased the number of stems as

concentration increased except at the highest concentration where stem number

decreased (Table 4.15). In Atlantic, the number of stems increased from 1.5 to 3 and 3.7

with 5 and 20 mg GA3/L respectively. However, a further increase in GA3 concentration

(to 40 mg/L) decreased stem number to 2.7. In Granola, the number of stems increased

from 2 to 2.9 with 5 and 20 mg GA3/L but at high concentration (40 mg/L) did not

promote increase the number of stems developed.

Table 4.15. Influence of GA3 on stem number per plant 69 DAP in Experiment 2 Means within rows and varieties followed by the same letter are not significantly different at P = 0.05

Variety GA3 (mg/L) 0 5 20 40

Atlantic 1.5a 3.0b 3.7c 2.7bd Granola 2.0a 2.9b 3.0b 2.5ab l.s.d. (P = 0.05) = 0.6

Combining GA3 and paclobutrazol together, generally, did not change stem number

compared to that of GA3 alone but stem number was higher than that of untreated plants

(data not shown).

Tuber number per plant at sampling

Generally paclobutrazol did not influence tuber number per plant at any sampling time

in either Experiment 1 (Table 4.16) or 2 (Table 4.17). The exception was in Experiment

99


1 at 74 DAP in Granola where tuber number did increase from 9.6 to 14.3 with 250 mg

paclobutrazol/L.

In Experiment 2, gibberellic acid increased tuber number per plant in Atlantic 69 DAP

(Table 4.17). In Atlantic, applied GA3 increased tuber number from 12.3 to 18, 21.7 and

19 with 5, 20 and 40 mg GA3/L respectively. Tuber number per plant in Granola was

not affected.

Table 4.16. Influence of paclobutrazol (PAC) on tuber number per plant at different harvest (days after planting) in Experiment 1. Means within columns and varieties followed by the same letter are not significantly different at P = 0.05.

Variety PAC Tuber number per plant at different harvest

(mg/L) 74 (DAP) 104 (DAP) Atlantic 0 12.3a 13.0a

100 10.2a 14.7a 250 9.8a 12.3a

Granola 0 9.6a 22.4a 100 9.6a 20.3a 250 14.3b 20.2a

l.s.d. (P = 0.05) 2.9 4.0

Table 4.17. Influence of paclobutrazol (PAC) and GA3 on tuber number per plant 69 days after planting in Experiment 2. Means within rows and varieties followed by the same letter are not significantly different at P = 0.05.

Variety Tuber number per plant with PAC (mg/L) 0 100 250 350 Atlantic 12.4a 13.7a 11.2a 10.3a Granola 19.7b 16ab 13.5a 16.7ab Tuber number per plant with GA3 (mg/L) 0 5 20 40 Atlantic 12.3a 18.0b 21.7b 18.9b Granola 19.7a 17.6a 22.2a 21.8a l.s.d. (P = 0.05) 4.3

100


Tuber number per plant at final harvest

Untreated plants grown in Manjimup over late spring through summer 2001

(Experiment 1) had 6.8 tubers/plant in Atlantic and 9.3 tubers/plant in Granola (Table

4.18). In 2002, in Perth, where plants were grown over late winter through spring

(Experiment 2), Atlantic produced 5.2 and Granola 7.7 tubers/plant (Table 4.19).

Tuber number in different size grades and in total weight yield was marginally

increased by paclobutrazol in a few instances. In Experiment 1 (Table 4.18) tuber

number in Atlantic was not influenced by paclobutrazol. In Granola, paclobutrazol

application increased the number of small (20-55 g) tubers from 2 to 2.8, but generally

the number of tubers in the larger size grades was not affected. Total tuber number in

Granola increased from 9.3 to 10.5 at 100 mg paclobutrazol/L. In Experiment 2 (Table

4.19), 250 mg paclobutrazol/L increased the number of 20-55 g tubers in Atlantic from

1.4 to 2. The number of tubers 56-110 g increased from 2.3 to 3.5 with 250 mg

paclobutrazol/L whilst the number of tubers 111-200 g decreased from 1.5 to 1.1 at

paclobutrazol concentrations applied. The total number of tubers increased from 5.2 to

6.8 with 250 mg paclobutrazol/L. In Granola, there were slight decreases in each tuber

size category but the total tuber number was not affected.

Gibberellic acid application shifted the size distribution of tubers toward a greater

proportion of small tubers at final harvest (118 DAP) (Table 4.20). The number of 20-

55 g tubers in Atlantic increased from 1.4 to 4 with 5 mg GA3/L, to 6 with 20 mg GA3/L

and to 5.4 with 40 mg GA3/L. The total number of tubers per plant increased with GA3

application, from 5.2 to 7.9, 9.1 and 8.3 with 5, 20 and 40 mg/L GA3 respectively. In

Granola, the of 20-55 g tubers per plant increased from 4.1 to 5 and to 5.6 with 20 and

40 mg GA3/L respectively. The number of tubers 56-110 g decreased from 2.9 to 2.1

and total tuber number increased from 7.7 to 9 per plant.

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Table 4.18. Influence of paclobutrazol on tuber number per plant in different size grades and total tuber number at final harvest 146 days after planting in Experiment 1. Means within columns and varieties followed by the same letter are not significantly different at P = 0.05.

Variety Paclobutrazol Tuber number per plant in different size grades (g) (mg/L) 20-55 56-110 111-200 201-250 >250 Total

Atlantic 0 1.0a 1.8a 2.2a 1.0a 0.7a 6.8a 100 0.8a 1.6a 2.6a 1.0a 0.8a 6.8a 250 0.8a 1.3a 2.6a 1.2a 0.9a 6.8a

Granola 0 2.0a 3.4a 2.9a 0.5ab 0.3a 9.3a 100 2.7b 3.4a 3.2a 0.7b 0.3a 10.5b 250 2.8b 3.4a 3.2a 0.4a 0.1a 10.0a

l.s.d. (P = 0.05) 0.4 0.5 0.4 0.2 0.3 0.9

Table 4.19. Influence of paclobutrazol on tuber number per plant in different size grades (g) and total tuber number at final harvest 118 days after planting in Experiment 2. Means within columns and varieties followed by the same letter are not significantly different at P = 0.05.

Variety Paclobutrazol Tuber number per plant in different size grades (g) (mg/L) 20-55 56-110 111-200 Total Atlantic 0 1.4a 2.3a 1.5b 5.2a 100 1.5a 2.5a 1.0a 5.0a 250 2.2b 3.5b 1.2a 6.8b 350 1.7ab 2.6a 1.0a 5.3a Granola 0 4.1b 2.9b 0.7b 7.7a 100 4.1b 2.5a 0.3a 7.0a 250 4.2b 2.8ab 0.2a 7.2a 350 3.5a 2.8ab 0.6b 7.0a l.s.d. (P = 0.05) 0.5 0.4 0.2 0.7

The combined treatment of GA3 and paclobutrazol increased the total number of tubers

developed compared with untreated plants and compared with paclobutrazol alone but

compared to GA3 alone, the number of tubers developed was decreased by the

combined treatment (Table 4.21). At 5 mg GA3/L, paclobutrazol reduced total number

of tubers per plant in Atlantic from 7.9 to an average of 5.9. At 20 mg GA3/L,

paclobutrazol reduced the total number of tubers from 9.1 to an average of 6.9. At 40

102


mg GA3/L, paclobutrazol decreased the number of tubers from 8.3 to an average of 6.9.

There was no interaction between GA3 and paclobutrazol on total tuber number in

Granola.

Table 4.20. Influence of GA3 on tuber number per plant in different size grades (g) and total tuber number at final harvest 118 days after planting in Experiment 2. Means within columns and varieties followed by the same letter are not significantly different at P = 0.05.

Variety GA3 Tuber number per plant in different size grades (g) (mg/L) 20-55 56-110 111-200 Total Atlantic 0 1.4a 2.3a 1.5d 5.2a 5 4.0b 3.0b 0.9c 7.9b 20 6.0d 2.6b 0.6b 9.1c 40 5.4c 2.6b 0.3a 8.3d Granola 0 4.1a 2.9b 0.7b 7.7a 5 4.1a 2.1a 1.1c 7.2a 20 5.0b 3.3c 0.6b 9.0b 40 5.6c 2.5a 0.2a 8.2bc l.s.d. (P = 0.05) 0.5 0.4 0.2 0.7

Table 4.21. Influence of GA3 and paclobutrazol (PAC) on total tuber number per plant at final harvest 118 days after planting in Experiment 2.

103



Tuber yield at final harvest

Atlantic and Granola grown in Manjimup (2001, Experiment 1) over late spring through

summer produced 82 t/ha (Table 4.22). Tuber size distribution of the two varieties was

quite different with a greater proportion of Atlantic tubers in the large grades than in

Granola. In Atlantic, the distribution of tuber size was 4.5% of 20-55 g, 17% of 56-110

g, 34% of 111-200 g, 23% of 201-250g and 22% of >250 g tubers, whilst in Granola,

the distribution of tuber size grades consisted of 9% of 20-55g, 29% of 56-110 g, 42%

of 111-200 g, 12% of 201-250 g and 8% of >250 g tubers.

Table 4.22. Influence of paclobutrazol on yield (t/ha) in different size grades (g) and total yield at final harvest 146 days after planting in Experiment 1. Means within columns and varieties followed by the same letter are not significantly different at P = 0.05.

104


Variety Paclobutrazol Tuber yield (t/ha) in different size grades (g) (mg/L) 20-50 56-110 111-200 201-250 >250 Total Atlantic 0 3.7a 13.9b 27.9a 19.0a 17.8a 82.3a 100 2.9a 11.7ab 26.7a 20.5a 23.4a 85.2a 250 2.9a 8.9a 31.3a 20.7a 23.0a 87.6a Granola 0 7.5a 23.8a 34.6a 9.8ab 6.5a 82.2a 100 9.7b 24.0a 38.1a 11.9b 6.4a 90.0b 250 10.5b 22.7a 36.4a 7.0a 3.0a 79.7a l.s.d. (P = 0.05) 1.8 3.3 7.4 3.6 6.7 7.6

In Perth (2002, Experiment 2) the untreated Atlantic and Granola, grown over late

winter and spring, produced 39 t/ha (Table 4.23 and Figure 4.2.)

Again, Atlantic had a higher yield of larger tubers than Granola. In Atlantic, total yield

was distributed with 12% of 20-55 g, 43% of 56-110 g and 45% of 111-200 g tubers. In

Granola, 32% of 20-55, 48% of 56-110 and 20% of 111-200 g tubers comprised the

total yield. There were no tubers harvested in large size grades (200-250 g and >250 g).

Table 4.23. Influence of paclobutrazol on yield (t/ha) in different size grades (g) and total yield in final harvest 118 days after planting in Experiment 2. Means within columns and varieties followed by the same letter are not significantly different at P = 0.05.

Variety Paclobutrazol Tuber yield (t/ha) in different size grades (g) (mg/L) 20-55 56-110 111-200 Total Atlantic 0 4.3a 15.9a 17.2c 38.5b 100 4.8 a 17.3a 13.0b 35.0b 250 7.7b 17.0a 13.1b 37.8b 350 5.4a 17.2a 9.8a 32.4a Granola 0 12.5a 18.5a 7.6c 39.0b 100 13.4a 16.8a 3.1a 33.4a

250 13.1a 17.8a 2.0a 33.0a 350 11.1a 18.0a 6.0 b 35.2a

l.s.d. (P = 0.05) 1.6 2.5 1.6 3.3

In Experiment 1, the distribution of tubers in the various size grades and the total tuber

yield shifted slightly with paclobutrazol application (Table 4.22). The yield of 20-55 g

Atlantic tubers was 3.7 t/ha and was not influenced by paclobutrazol application. The

105


yield of 56-110 g tubers decreased, from 13.9 to 8.9 t/ha with 250 mg paclobutrazol/L

application. The yield of larger sized tubers (i.e. 111-200, 201-250 and >250 g) was not

influenced. The total yield was 82 t/ha and was not influenced by paclobutrazol

application. In Granola, the yield of small tubers (20-55 g) increased from 7.5 to 10 t/ha

at all paclobutrazol concentrations applied and the total tuber yield increased from 82 to

90 t/ha when 100 mg paclobutrazol/L was applied.

In Experiment 2 (Table 4.23), the yield of small, Atlantic (20-55 g) tubers increased

from 4.3 to 7.7 t/ha when 250 mg paclobutrazol/L was applied, however the yield of 56-

111 g potatoes was not affected and yield of 111-200 g potatoes decreased from 17.2 to

13 t/ha with 100 and 250 mg paclobutrazol/L. This was further decreased to 9.8 t/ha

with 350 mg paclobutrazol/L. Total yield decreased from 38.5 t/ha to 35 t/ha with 100

mg paclobutrazol/L and it further decreased to 32.4 t/ha with 350 mg paclobutrazol/L.

In Granola, yield of 111-200 g tubers reduced from 7.6 to 3 and 2 t/ha with 100 and 250

mg paclobutrazol/L respectively. Total yield decreased from 39 to an average of 33.6

t/ha across paclobutrazol concentrations.

The application of gibberellic acid shifted the size distribution of tubers towards a

greater portion of small tubers as concentrations increased (Figure 4.2). In Atlantic,

yield of 20-55 g potatoes increased from 4.3 to 12.8, 18.4 and 16.7 with 5, 20 and 40

mg GA3/L respectively. Yield of 56-110 g potatoes increased from 15.9 to 20.2 t/ha

with 5 mg GA3/L. Yield of 111-200 g potatoes decreased from 17.2 to 9.6, 6.8 and 3.0

t/ha with 5, 20 and 40 mg GA3/L respectively. With 20 mg GA3/L, the size distribution

of Atlantic tubers was composed of 45% of 20-55 g tubers, 38% of 56-110 g tubers and

17% of 111-200 g tubers with no affect on total yield. The total tuber yield increased

from 38.4 to 42.7 with 5 mg GA3/L but it decreased to 34.3 t/ha with 40 mg GA3/L.

In Granola, tuber yield also shifted with GA3 application to a greater portion of small

tubers (Figure 4.2). Although 5 mg GA3/L did not influence the yield of 20-55 g tubers,

106


higher concentrations increased it from 12.5 to 15.2 t/ha with 20 mg GA3/L and to 16.8

with 40 mg GA3/L. The yield of 56-110 g potatoes increased from 18.5 to 21.1 t/ha with

20 mg GA3/L but it decreased to 15.1 t/ha with 40 mg GA3/L. Yield of 111-200 g

decreased from 7.6 to 5.2, 6 and 2 t/ha with 5, 20 and 40 mg GA3/L respectively. Total

yield increased from 39 to 43 t/ha with 20 mg GA3/L but it decreased to about 34 t/ha

with both 5 and 40 mg GA3/L. Elongated, bent and heart shaped tubers were observed

at 40 mg/L GA3-treated plants in Atlantic and ≥20 mg GA3/L in Granola.

Atlantic

Tube

r yie

ld (t

/ha)

0

10

20

30

40

50 Control5 mg GA3/L20 mg GA3/L40 mg GA3/L

Granola

Tuber size grade (g)

20-55 56-110 111-200 Total

Tube

r yie

ld (t

/ha)

0

10

20

30

40

50

Figure 4.2. Influence of GA3 on yield (t/ha) in different size grades (g) and total yield at final harvest 118 days after planting in Experiment 2. Vertical bars are l.s.d. values at P = 0.05.

107


There were some negative interactions between GA3 and paclobutrazol treatments on

total yield (Table 4.24). Generally, total yield decreased with the combined GA3 and

paclobutrazol treatments as compared with GA3 alone. In Atlantic, total yield was 42.7

t/ha with 5 mg GA3/L and this was reduced to 28-33 t/ha with addition of paclobutrazol.

At 20 mg GA3/L the total yield was 40.7 t/ha and this was reduced to 28-34 t/ha with

addition of paclobutrazol. At 40 mg GA3/L yield was 34 t/ha and this decreased to 26

t/ha with addition of 250 mg paclobutrazol/L. In Granola, interactions occurred with 20

mg GA3/L and total yield decreased from 43 t/ha to 33-35 t/ha with applied

paclobutrazol.

Table 4.24. Interaction of GA3 and paclobutrazol (PAC) on total yield (t/ha) at final harvest 118 days after planting in Experiment 2.

Variety GA3 Paclobutrazol (mg/L) mg/L 0 100 250 350 Atlantic 0 38.5 35.0 37.8 32.4 5 42.7 33.3 30.4 27.8 20 40.7 34.0 28.2 30.3 40 34.3 32.7 25.8 30.4 Granola 0 39.0 33.4 32.9 35.2 5 33.5 33.0 31.1 30.6 20 43.0 34.7 32.6 33.6 40 34.0 32.5 32.3 35.9 l.s.d. (P = 0.05) Variety x GA3 = 3.3 Variety x PAC = 3.3 Variety x GA3 x PAC = 6.7

4.4. Discussion

The two varieties, Atlantic and Granola, had contrasting growth habits. Although total

yield (t/ha) was similar, Atlantic had less shoots and developed only a few large tubers

compared with Granola. These factors influenced the response of these varieties to the

growth regulator treatments.

108


Experiment 1 was planted in Manjimup in mid-spring whereas Experiment 2 was

planted in Perth in late winter. Generally, days were cooler in Manjimup (21oC) than

Perth (24oC). Low temperatures promote tuber initiation both under field (Midmore

1984; Midmore et al. 1986a) and control conditions (Borah and Milthorpe 1962). The

cooler weather in Manjimup should promote tuber initiation and increased tuber number

resulted in 31% more tubers in Atlantic and 21% more tubers in Granola compared to

Perth planting.

Yield (t/ha) in Perth was half that of Manjimup. Again, high temperature in Perth might

be one factor responsible for low yield. Assimilate partitioning to tubers reduces at high

temperatures but it increases to other parts such as stems and shoots (Ewing 1981; Wolf

et al. 1990). High temperatures increase GA synthesis in buds and this inhibits

tuberization (Menzel 1983). Application of paclobutrazol might be not able to

counteract the effect of endogenously synthesized GA stimulated by high temperature

in Perth. This may partly explain the lower yield response of paclobutrazol-treated

plants grown in Perth (Experiment 2) compared to Manjimup.

Overall growth of potato plants is reduced at high temperatures (Ewing 1981). The high

temperatures (30oC) in December in Perth might have reduced plant growth and in

addition, promoted early senesce which reduced the growing period and presumably

yield. Yield depends on length of growing period, the amount of foliage to intercept

light and photosynthetic rates of foliage (van der Zaag and van Loon 1987).

Therefore, potatoes grown under the high temperature of Perth had less foliage, a

shorter growing period and yielded less than potatoes grown in Manjimup, which had

lower temperatures across the growing period and plants there had more foliage and a

longer growing period.

Effects of paclobutrazol

109


Paclobutrazol retarded shoot growth. It reduced shoot dry weight and decreased plant

height due to a reduction in internode length. Similar responses occurred in potatoes

under glass house conditions (Balamani and Poovaiah 1985; Bandara et al. 1998).

Paclobutrazol did not influence stolon length but it reduced stolon and root dry weights,

which indicated an overall reduction in stolon and root growth. Suppression of shoot,

stolon + root growth in potato is known to be influenced by other growth retardants

such as CCC (2-chloroethyl-trimethylammonium chloride) under pot culture (Langille

and Hepler 1992; Abdala et al. 1995) and field conditions (Sharma et al. 1998b).

The application of paclobutrazol increased leaf chlorophyll content per unit area but did

not influence leaf area of fully expanded leaves. This confirms earlier reports of a

higher leaf chlorophyll content after paclobutrazol (Balamani and Poovaiah 1985) and

other growth retardant application (Sharma et al. 1998a; Sharma et al. 1998b; Sekhon

and Singh 1985). The higher chlorophyll content of paclobutrazol-treated plants may

benefit photosynthetic rate (Kumar et al. 1980; Nemchenco et al. 1981).

Paclobutrazol had an inconsistent and small effect on total tuber number per plant. Total

tuber number of Atlantic potatoes was not influenced by paclobutrazol in Experiment 1

but it increased in Experiment 2. Paclobutrazol increased total tuber number in Granola

in Experiment 1 but had no influence in Experiment 2. Paclobutrazol blocks GA

biosynthesis (Rademacher 1999) and this maintains low endogenous GA levels, which

is a prerequisite for tuber initiation (Koda and Okazawa 1983; Xu et al. 1998b).

Paclobutrazol can increase tuber number under tissue culture (Simko 1993; Simko

1994) and mini-tuber production under pot culture (Bandara and Tanino 1995; Bandara

et al. 1998). Paclobutrazol also enchances the flow of assimilates to storage organs such

as the corms of gladiolus cultured in vitro (Steinitz et al. 1991; Simko 1993) and in

potato it may direct assimilate away from shoots and toward tubers. Similarly, in fruit

trees, such as apple, paclobutrazol alters assimilate partitioning from vegetative shoots

110


to reproductive organs such as buds, flowers and fruits and this increases yield (Lever

1986). In my experiments, paclobutrazol-treated potatoes were expected to increase the

number of tubers but this was not consistently indicated by the results. It was hoped that

this could be transferred to the field. However, growth is much more variable under

field compared with controlled conditions. Determining the time of tuber initiation is

difficult in the field (Cho et al. 1983b; Ewing and Struik 1992). It is difficult to observe

tuberization in the field and requires frequent sampling of plants. These two factors

made field assessment of the stages of tuberization very difficult and thus the

application of paclobutrazol was less accurate.

There was a great deal of variability in the yield of small round seed potatoes with

paclobutrazol. The yield of small round seed of Granola increased by 40% in

Experiment 1 but had no effect in Experiment 2. Paclobutrazol did not influence yield

of small round seed of Atlantic in Experiment 1 but increased it by 80% in Experiment

2. The timing of paclobutrazol application at the correct development stage is critical.

The stage chosen for paclobutrazol application in this study was during early tuber

initiation when swelling tubers were about twice that of the stolon diameter (Firman et

al. 1991). This stage is critical as it determines potential yield (Ewing and Struik 1992;

Ewing 1995). Tuber initiation is prevented, inhibited or delayed by endogenous

gibberellins (Vreugdenhil and Struik 1989; Abdala et al. 1995) and application of anti-

gibberellins, such as paclobutrazol at early tuber initiation should reduce synthesis of

gibberellins and thereby promote tuberization (Simko 1993). Tubers were initiated at

different times and variation existed within and between rows. Treatment was probably

too late to block synthesis of new endogenous gibberellins, which were then able to

inhibit tuberization in many plants.

In terms of total tuber yield, the influence of paclobutrazol was similar to that on the

small round seed potatoes (had an inconsistent effect on total yield). It did not influence

111


total yield of Atlantic in Experiment 1 and decreased yield in Experiment 2. It

increased total yield in Granola in Experiment 1 but it did not influence it in Experiment

2. Increased yield can be gained through improved translocation of assimilates from

shoot to tubers (Sekhon and Singh 1984). However, the increase in yield of potato

tubers was inconsistent between the two experiments and so no conclusions could be

drawn for the role of paclobutrazol in these varieties.

Genotypes might interact with environment. Granola may need more favourable

conditions for tuberization, such as lower temperature, higher moisture and better soil

conditions. Atlantic is probably more tolerant to adverse environmental conditions.

Effects of GA3

Applied GA3 hastened first and complete plant emergence for both Atlantic and

Granola. The higher the GA3 concentration the faster plants emerged and 40 mg GA3/L

reduced the time to first plant emergence by one week for both varieties and complete

emergence by 4 days in Atlantic and 2 days in Granola. The faster emergence was

probably due to rapid shoot elongation induced by the GA3 treatments (Holmes et al.

1970). In many plants, GA3 promotes cell elongation and hence rapid shoot elongation

(Metraux 1987; Jacobsen et al. 1995).

Gibberellic acid-treated plants were taller and weighed more than untreated plants.

Treated plants were twice the height of untreated plants due to increased internode

length, as has been found earlier (Sharma et al. 1998b). Gibberellic acid increases shoot

sink strength and this may have lead to the increased shoot dry weight (Menzel 1980;

Sharma et al. 1998b).

Chlorophyll content per leaf area decreased with GA3, which is similar to earlier reports

(Agarwal et al. 1983; Sharma et al. 1998b). This was probably because chlorophyll

biosynthesis is regulated by GA (Mathis et al. 1989; Jacson and Prat 1996).

112


Stem number increased with a pre-treatment of GA3 to a maximum number and then

decreased with the higher concentration (40 mg/L). The optimum GA3 concentration for

maximum stem number in both Atlantic (3.7 stems) and Granola (3 stems) was 20

mg/L. Concentration of GA is important in determining the magnitude of the stem

response.

Concentrations of GA3 required for maximum stem number probably depend on the

apical dominance of the varieties. It seems that varieties with more apically dominant

growth require higher GA concentrations. Atlantic was more apically dominant than

Granola. This is supported by an earlier investigation (Harrington 2000) where Atlantic

had strong apical dominance though data about Granola were absent. At the same

chronological age of 4 months storage at 4oC, only the apical sprout is present in

Atlantic whilst lateral sprouts are present in Granola. In other varieties, such as Alpha,

which emerge slowly probably due to strong apical dominance, a concentration of 45

mg GA3/L is required for maximum stem number (Marinus and Bodlaender 1978) and

Majestic requires 100 mg/L (Holmes et al. 1970).

Treating seed with GA3 before planting increased tuber number per plant probably

through increased in stem number in Atlantic but not in Granola. There is frequently a

positive correlation between stem number and tuber number per plant in GA-treated

potatoes (Holmes et al. 1970; Marinus and Bodlaender 1978; Sekhon and Singh 1984).

The below ground stem is the site of stolon initiation (Wurr et al. 1997). Stolon number

is closely related to stem number and generally the more stems possessed by the plant

the more stolons are produced (Svensson 1962). The sub-apical region of stolon tips are

the sites of tuber initiation (Koda and Okazawa 1983; Vreugdenhil and Struik 1989; O'

Brien et al. 1998; Xu et al. 1998a; Jacson 1999). Thus, more stems produce more

stolons and this provides more tuber initiation sites which, in turn, lead to greater

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numbers of tubers. This at least partly explains the positive correlation between stem

number and tuber numbers (Haverkort et al. 1990b; Haverkort et al. 1990c).

Stolon branching caused by GA3 treatment, as observed in the present experiment,

might also contribute to the high tuber number, as reported earlier (Bodlaender and van

de Waart 1989). This provides more sites for tubers to initiate (Struik et al. 1988;

Bodlaender and van de Waart 1989; Gill et al. 1989).

Further research is required to elucidate the mechanism of increasing tuber number

through increased stem number in Atlantic and Granola with GA3. Stem number per

plant, stolon number per stem and tuber numbers per stolon measurements are required

to determine the source of additional tubers. The influence of GA3 on stolon branching

also needs to be investigated and to what extent this contributes to increases in tuber

number.

Increased tuber number with GA3 in Granola was not as great as for Atlantic. With 20

mg GA3/L tuber number increased from 5 to 9 in Atlantic and from 8 to 9 in Granola

and increasing concentration to 40 mg/L reduced tuber number. It seems these were the

maximum tuber numbers for Atlantic and Granola with short growing period (118 days)

although with longer growing period (146 days) maximum tuber number in Granola

was higher (9.3 tubers/plant) (Table 3. 16). In other varieties, such as Ranger Russet

and Shepody, maximum tuber number with GA3 treatment and with long growing

period are lower than Atlantic and Granola at 7.5 and 6 respectively (Mikitzel 1993).

In Granola, the effect of increasing stem number on tuber number was not as great as in

Atlantic. This was probably because Granola naturally produces more tubers per stem

than Atlantic so Granola had lower capacity to increase stem number with GA3 than

Atlantic.

Finding treatments to increase number of small round seeds in varieties that naturally

produces a high number of tubers such as Granola is difficult. Untreated Atlantic

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produced 5 tubers whilst Granola produced 8 tubers and application of 20 mg GA3/L

increased tubers number to 9 for both varieties. There is very little chance for varieties

that naturally produce high numbers of tubers to further increase their tuber number

with GA3 possibly because the limit for tuber production has been reached.

Shifts in yield profiles toward a greater yield of small tubers with GA3 treatment may be

more likely in varieties that naturally produced a high proportion of large tubers and

less likely to occur in varieties that naturally produce a high proportion of small tubers.

The proportion of untreated Atlantic tuber that were 20-55 g was 12% compared with

32% in Granola and these changed to 45% in Atlantic and 30% in Granola with 20 mg

GA3/L. Potato varieties, which naturally produce large tubers, or a greater proportion of

large tubers can generally be shifted with GA toward tuber size profiles with an

increasing yield of small tubers (Marinus and Bodlaender 1978; Sekhon and Singh

1984; Mikitzel 1993).

Increasing stem and tuber number imposes a high inter-stem and inter-tuber competition

for nutrients, water and light and this reduces the average size of each tuber (Moorby

1967; Bishop and Timm 1968). In the present experiment, where tuber number

increased with GA3, these tubers competed for limited assimilates resulting in a higher

proportion of small tubers.

Concentration of GA3 used is an important factor determining tuber number. A

concentration of 20 mg/L was optimum for Atlantic and Granola. However deformed

tubers occurred in Granola at 20 mg/L. Varieties vary in their tuber number response to

applied GA3 and perhaps only limited ranges of concentrations are suitable to increase

tuber number. If GA3 concentrations are too high tuber number will decrease, if too

low the maximum tuber number cannot be achieved. Yield of Kufri Chandramukhi

potatoes decreases with 100 mg GA3/L (Sharma et al. 1998b) lower GA3 concentrations

115


(10 and 20 mg/L) increase tuber number (Sekhon and Singh 1984). Similarly, with

Atlantic and Granola, tuber number decreased with high GA3 concentration (40 mg/L).

Concentrations of GA3 that were too high deformed the tubers. Heart-shaped tubers

were observed in Atlantic with 40 mg GA3/L whilst they were bent and elongated in

Granola with 20 and 40 mg GA3/L. Deformed tubers also occur in other varieties

treated with high GA concentrations, such as Up-to-Date potatoes with 50 mg GA/L

(Slomnicki and Rylski 1964) and White Rose with ≥ 5 mg GA/L (Timm et al. 1960).

Gibberellic acid may influence tuber filling (Sharma et al. 1998b). Deformities

probably occur because gibberellic acid reduces sink strength of tubers and reduces

starch deposition (Lovell and Booth 1967; Booth and Lovell 1972; Mares et al. 1981)

and tuber filling (Sharma et al. 1998b). A reduction of starch deposition into tubers

would influence tuber growth and the hook of swelled stolons may fail to straighten due

to insufficient starch to fill tubers, resulting in bent tubers.

Water deficit can lead to malformation of tubers (Iritani 1981). Inadequate irrigation

during the early growing season probably caused elongated and pointed tubers. The

low water-holding capacity of the sandy soil in Perth meant more frequent irrigation

was required. It is suspected that water deficit at early tuber initiation was the major

cause of malformation.

High temperatures during plant growth also deform tubers (Caldiz 1996). Mean

maximum temperature in Perth across the growing session was 24oC with the highest

maximum temperature of 30oC in December. High temperatures experienced during

production of Atlantic and Granola may have deformed tubers.

High GA concentrations can reduce total yield. In the present experiment, 40 mg GA3/L

caused a 12% reduction of total yield in Atlantic and 14% in Granola. It decreases total

yield in Kufri Chandramukhi with 100 mg GA/L (Sharma et al. 1998b). However in

Keswick 8 mg GA/L is too high to maintain total yield (Smeltzer and Mackay 1963).

116


This is probably because potato varieties have different sensitivities to GA

concentrations (Mikitzel 1993).

The optimum GA3 concentration for maximum tuber number, total tuber yield and yield

of small round seeds without deformed tubers was 20 mg/L for Atlantic and probably

between 5 and 20 mg/L for Granola. Concentration of 5 mg GA3/L was too low for

Granola and unable to increase tuber number and 40 mg/L was too high and caused

deformation of tubers and reduction in total yield. Further research is necessary to

investigate the optimum GA3 concentrations for Granola, which is probably between 10

and 20 mg/L.

Timing of GA application is critical for increasing tuber numbers. Application after

storage and before planting was effective in increasing tuber number. At this time,

potatoes had just broken dormancy and exhibited apical dominance, especially in

Atlantic. Application of GA reduces apical dominance and allows multiple sprouts and

stem growth (Timm et al. 1962; Holmes et al. 1970). More stems might produce more

stolons and these provide more tuber initiation sites and more tubers.

Method of GA application also influences the response. Dipping cut seed was effective

in increasing stem and tuber number as has been reported elsewhere (Slomnicki and

Rylski 1964; Mikitzel 1993). Cut seeds readily absorb GA (Sekhon and Singh 1984).

Although foliar spray (Caldiz 1996; Caldiz et al. 1998) and soil application (Struik et al.

1989b) of GA are effective in increasing tuber number and shifting tuber size

distribution toward a greater yield of small tubers, the dipping method is probably more

convenient for a production system. Dipping is not dependent on weather. Absorption

of GA is more uniform and more efficient with the dipping method compared with soil

or foliar application methods.

Gibberellic acid substantially increased yield of small round seed and decreased yield of

bigger sized tubers. The 4-5 fold increase in small round seed in Atlantic indicated that

117


GA3 was a powerful plant growth regulator for reducing seed size in tubers. The shift in

tuber size distribution towards a greater proportion of small tubers appeared to be due to

increased stem number. Bleasdale (1965) also found that tuber size distribution is a

function of stem number.

Starch and total sugar contents were not influenced by GA3. This indicates that potential

vigour for subsequent growth was not affected by GA3, as starch is the main

carbohydrate reserve in potatoes (Oparka 1986).

Seed needs to be of high quality (Struik and Wiersema 1999). Internal observation

revealed that there were no disorders such as hollow heart or black heart in Atlantic and

Granola tubers with GA3 treatments.

Application of GA should not have any carry-over effects on subsequent plant growth

and development (Bishop and Timm 1968; Holmes et al. 1970; Bodlaender and van de

Waart 1989). This needs further investigation using seeds from the present experiments.

Effect of combination of GA3 and paclobutrazol

Generally, GA3 and paclobutrazol had opposite effects on shoot and stolon growth.

Paclobutrazol alone did not consistently increase tuber number and yield of small round

seed. Therefore, no positive interaction was observed. Timing of paclobutrazol

application may be critical to target the exact stage of tuber initiation, which was

difficult to determine under field conditions. It seems there was a narrow window of

opportunity for application of paclobutrazol. The failure of combined treatment of GA3

and CCC for increasing tuber number, in pot grown potatoes, was probably because of

high GA3 concentration (50 mg/L) and then CCC applied at an inappropriate time. This

is likely related to the fact that CCC was applied after shoots had emerged without any

indication of tuberization stage (Dyson 1965). Under controlled environmental

conditions combining GA3 and an anti-gibberellin CCC (2-chloroethyl

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trimethylammonium chloride) increased tuber number (Kumar and Warieng 1974) but it

was not possible to repeat this under the field conditions employed here. The high

concentration (1000 mg/L) of CCC used was probably effective in counteracting effects

of applied GA3 at 10 mg/L. In the present experiment concentration of GA3 was not the

factor causing the failure of combined treatment of GA3 and paclobutrazol in increasing

tuber number because GA3 alone increased tuber number considerably. Instead, it was

caused by inappropriate time of paclubtrazol, which was possibly too late.

4.5. Conclusions

The application of gibberellic acid to seed tubers induced early plant emergence,

reduced chlorophyll content and promoted shoot and stolon growth. Treated plants were

twice the height of untreated plants due to increased in internode length. Giberrellic acid

also increased tuber number per plant by 75% in Atlantic and 17% in Granola through

increased stem number. .

The distribution of tuber size shifted with GA3 application. Increased numbers of tubers,

induced by high competition between tubers for limited assimilates, resulted in smaller

tubers. Consequently, the yield of small round seeds (20-55 g) increased whilst the yield

of bigger tubers decreased without reducing the total yield. The application of

gibberellic acid significantly increased the yield of small round seeds (20-55 g) in

Atlantic (by 328%) and to a lesser extent in Granola (22%). According to the results

application of GA3 would benefit commercial seed producers cultivating Atlantic and

Granola potatoes under field conditions in Western Australia.

Response of potato plants to applied GA3 varied with variety, concentration, time and

method of application. Granola naturally produced large numbers of small tubers and it

is likely that further increases in tuber number will be difficult to obtain. A shift towards

119


a greater proportion of small tubers is easier to produce in Atlantic, which naturally

produces a few, larger tubers.

The optimum concentration to increase tuber number per plant, total yield, and yield of

small round seeds was 20 mg/L for the concentrations examined in both Atlantic and

Granola. However, at 20 mg GA3/L shape abnormalities were observed in Granola and

therefore lower concentrations, between 5 and 20 mg/L should be examined if GA3 is to

be used to improve yield of small round seed in commercial production. Time and

method of GA3 application are important considerations for seed potato production.

Gibberellin should be applied before planting to relatively young seeds when seeds

exhibit apical dominance. Dipping cut seeds for 30 minutes in GA, 2 days before

planting was most effective, and operationally, this may be more convenient for

farmers.

Application of paclobutrazol at early tuber initiation reduced stolon and root dry

weights, shoot dry weight (only in Atlantic), plant height and internode lengh but it

increased chlorophyll content of leaves. Total tuber number per plant and yield of small

round seeds were inconsistantly effected by paclobutazol where it either increased or

had no influence. The inconsistancy was probably because timing of paclobutrazol

application was too late. This relates to variability in time of tuberization between and

within rows so it was difficult to determine time of early tuber initiation under field

conditions.

Combined treatment of GA3 and paclobutrazol reduced tuber number and total yield

compared to that of GA3 alone probably due to inappropriate timing of paclobutrazol

application.

4.6. Recommendations

Gibberellic acid is a powerful growth regulator for increasing the yield of small round

seeds (20-55 g) in Atlantic and probably can be used in commercial seed potato

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production. Gibberellic acid is available in commercial formulations for use in food

production, such as table grapes, mandarin and orange by Aftern Ltd, Perth under the

trade name of Grando GA3. Use in seed potato production requires registration but this

should be possible. In addition GA3 usually has no carry- over effects on subsequent

potato growth (Bishop and Timm 1968; Holmes et al. 1970; Bodlaender and van de

Waart 1989). Gibberellin is used to produce seeds, which are a generation away from

potatoes for human consumption and GA3 naturally occurs in potatoes (Xu et al. 1998b;

Abdala et al. 2002; Berg et al. 1995). These factors should further ease the registration

process. Increasing small round seed potatoes with GA3 will increase export to

Indonesia and other Southeast Asian countries and will support potato production in

Indonesia.

It could be recommended that GA3 be applied as a dip for seed pieces 2 days before

planting at 20 mg/L for Atlantic. The dip method is more convenient for commercial

production because applications are not dependent on weather. Atlantic seeds stored for

intermediate periods (4 months at 4oC), which had broken endodormancy and

ecodormancy but were still apically dominant, responded well to applied GA3.

After intermediate storage period Granola had broken dormancy and was no longer

apically dominant and so the response to GA3 was small but it still increased yield of

small round seeds without reducing total yield. Therefore no recommendation can be

made for the use of GA on this variety at this time.

121

General discussion

122

Chapter 5

General Discussion

Tuber size distribution is an important characteristic in potato production. The desired

size depends on its use and the market demands. The profit is related to maximizing the

production of the desired sizes. Atlantic and Granola are two important varieties in

Indonesia. Small seed tubers weighing 20-55 g are required for seed potatoes in

Indonesia and maximizing their yield in both varieties was the main target of this thesis.

Atlantic naturally produces fewer, larger tubers and these are not suitable for use as

whole seeds. Granola naturally produces mostly smaller tubers but it still produces a

few in the larger size grades. However, there is no fresh market for the large Granola

tubers in Western Australia. Currently, the insufficient yield of small tubers and low

profitability limits export of these varieties to Indonesia. In order to lift profit for export

sales methods of reducing tuber size, without reducing total yield, are required for

Western Australian seed growers.

Three approaches were examined. The first approach used was to increase stem number,

which would increase tuber number (Allen and Wurr 1973). High tuber number induces

high inter-tuber competition for assimilates and this reduces tuber size (Moorby 1967;

Bishop and Timm 1968). The second approach was to improve carbohydrate

partitioning toward tubers to maintain as many tubers as initiated. The third approach

was combining the first and the second approaches.

Physiological age of tubers can be modified by prolonging the time in cool storage (van

der Zaag and van Loon 1987). Physiological age can influence the numbers of sprouts

that develop and hence increase the number of stems (Hartmans and Van Loon 1987).

Sprout number increased with increasing storage duration at 4oC and the highest sprout

number was achieved after 28 - 30 weeks storage in Atlantic and 26 weeks in Granola.

As the physiological age of tubers increased with prolonged storage so was their apical

General discussion

123

dominance gradually lost. A reduction in apical dominance released lateral sprouts from

correlative inhibition and resulted in multiple stems (Bodlaender and Marinus 1987;

Mikitzel and Knowles 1989a; Mikitzel 1990; Knowles and Bottar 1991). Storage

duration at 4oC generally did not influence stem number in Atlantic whole seeds except

duration of 30 weeks decreased stem number. In Granola whole seeds stem number

after storage for 22, 24, 26 and 30 weeks was same but they were higher than stem

number after 22 weeks storage. Whilst storage duration is slightly effective it is not

always practical. Many growers store potatoes in central facilities. Storage facilities

must be locally available, cost efficient and the duration must meet other seasonal and

scheduling arrangements of the grower.

Apical sprout removal also modifies stem number (Hay and Hampson 1991). In

Granola, apical sprout removal increased stem number and in turns tuber number and

yield. Apical sprouts are sites of auxin synthesis (Menzel 1981) and auxin is transported

basipetally to enter lateral buds where it acts to inhibit the outgrowth of lateral buds

(Kumar and Knowles 1993). Removing apical sprouts removes the source of correlative

inhibition of lateral sprouts. In this study apical sprout removal in Atlantic did not

influence stem number although it has been reported to increase stem number

(Harrington 2000). The discrepancy was probably related to differences in the number

of sprouts removed. Harrington (2000) removed all the sprouts at the rose end, whilst in

the present experiment only the longest sprout at the rose end were removed. The

removal of only the longest sprouts may have been insufficient for reducing apical

dominance, which may have been maintained by the remaining sprouts at the rose end.

The advanced stage of sprout growth (5 cm long) when they were removed (Harrington

2000) indicated an advanced physiological age of seeds and this probably also

influenced stem number response to apical sprout removal. Old seeds probably give

more stems with apical sprout removal. In the present experiment apical sprout removal

General discussion

124

was carried out when seeds had 2-4 mm long sprouts. Although apical sprout removal

was effective in Granola it was not a practical method.

Perhaps a more practical approach to remove sprouts and increase the number of stems

is by applying chemicals that burn dominant sprouts and thus stimulate lateral sprout

growth. Carvone is a natural sprout suppressant for potato during storage (Beveridge et

al. 1981a; Sorce et al. 1997; Hartmans and Oosterhaven 1998). Carvone reduces apical

dominance (Oosterhaven et al. 1995; Baker et al. 2002) by physically damaging sprouts

(Baker et al. 2002) and increases lateral sprout growth and the number of stems

developed (Hartmans and Oosterhaven 1998; Brown et al. 2000). In the present

investigation, carvone applied to Atlantic and Granola did not increase the number of

stems developed or total tuber number probably becuase lower headspace concentration.

This was probably related to the simple method of carvone application where the

method could not create aptimum and constant headspace concentration. Alternatively,

the time of carvone application probably was not appropriate. The sprouting stage when

carvone should be applied in Atlantic and Granola is not known, therefore it requires

further investigations.

In artificial systems, such as tissue culture, paclobutrazol have been used to promote

tuber initiation and enhance tuber growth by redirecting assimilates away from shoots

toward tubers (Simko 1991). The use of paclobutrazol, to promote tuberization in the

field was investigated. It was found that paclobutrazol application increased the number

of tubers developed by Atlantic and Granola but the results were variable. In the first

experiment, the number of tubers increased in Atlantic but not Granola. In the second

experiment the number of tubers increased in Granola with no effect in Atlantic. There

is some evidence in the literature that paclobutrazol application leads to the

development of increased numbers of tubers in tissue culture (Simko 1991; Simko

1993; Simko 1994) and pot-grown potatoes (Bandara et al. 1998; Bandara and Tanino

General discussion

125

1995). This effect on tuber number is thought to result from a reduction in the levels of

endogenous GA in stolons, the sites of tuber initiation (Hammes and Nel 1975; Davis et

al. 1998). This presumably did not occur consistently in my experiments.

The influence of paclobutrazol on tuber yield was also inconsistent. In the first

experiment, paclobutrazol application increased the yield in Granola but not Atlantic. In

the second experiment, paclobutrazol application decreased the yield of both Atlantic

and Granola. In this study, my aim was to apply paclobutrazol during early tuber

initiation. Determining the timing of tuber initiation in the field is difficult because

tubers initiate underground and cannot be observed without disturbing them. Frequent

hand excavation of tubers is required in order to assess the stage of tuber development

and this possibly destroys some roots, stolons and tubers (Cho et al. 1983b; Ewing and

Struik 1992; Helder et al. 1993a). Therefore, part of the variation in tuber number and

yield after paclobutrazol application may be related to differences in its effectiveness at

the different stages of tuber development. The variation in stolon growth and tuber size

within and between plants was high and it is likely that estimation of early tuber

initiation was not sufficiently accurate. It is well known that paclobutrazol can redirect

assimilate away from shoots toward tubers (Balamani and Poovaiah 1985). The results

presented here partly support that claim, at least for Granola. However, tuber yield was

not always increased in my field-grown plants and clearly, if the response to

paclobutrazol was aimed at directing assimilates toward tuber growth, large variations

resulted in poor overall performance. Based on my findings it is therefore concluded

that the use of paclobutrazol as a foliar spray is not likely to provide a practical option,

because the response greatly depends on the timing of application (stage of tuber

development) and this is difficult to assess under field conditions. In addition, spraying

paclobutrazol depends on weather and it could be delayed by rainy and windy weather.

General discussion

126

The use of herbicides is practical for weed control and paraquat + diquat are herbicides

registered for potato. Paraquat + diquat in the formulation of Spray.Seed® can wilt

leaves (Summers 1980; Ashton and Monaco 1991) and at lower rates this might

suppress shoot growth and promote tuber growth by redirecting assimilates to tubers.

Young shoots might be the most susceptible to low rates of paraquat + diquat and the

death of these shoots might reduce GA biosynthesis and reduce GA transport to stolons

thereby promoting tuber initiation. Initiated tubers are normally maintained by

assimilate supply and if more assimilates were redirected to more tubers most of them

would grow to reach marketable seed size. In this study, paraquat + diquat did not

promote tuberization nor redirect assimilates to tubers. The herbicides reduced leaf

chlorophyll content and photosynthesis (Summers 1980) thus reducing yield,

particularly when applied at 500 mL/ha during early tuber initiation. It is concluded that

both low (500 mL/ha) and very low (250 mL/ha) rates of paraquat + diquat, are not

useful for increasing tuber number, total yield and yield of small round seed potatoes.

Gibberellic acid promotes stem growth (Mikitzel 1993). It increased stem number from

1.5 to 3.7 in Atlantic and from 2 to 3 stems per plant in Granola. Stem number per seed

piece and number of seed pieces planted was used to determine stem density, which

increased from 124, 999/ha to 308,332/ha with GA3 in Atlantic. Similarly in Granola,

stem density increased from 166, 666/ha to 249,999/ha with GA3. High stem density

leads to the development of greater inter-stem competition for light, minerals and water

and these effectively decrease stem size. In turn, the amount of foliage to supply

carbohydrate for tubers decreased and this reduced tuber size (Moorby 1967).

Gibberellic acid increased tuber number more effectively in Atlantic than in Granola.

This was probably due to varietal differences, as Atlantic naturally produces fewer and

larger tubers than Granola. The magnitude of response to GA3 for increasing tuber

number was greater in the variety that naturally produces larger, fewer tubers.

General discussion

127

The combination of high stem density and high tuber number tended to reduce tuber

size and shift tuber size distribution toward a greater proportion of small tubers. The

application of GA3 was found to be useful for increasing small tuber yield and this

technique could benefit seed potato growers. However, registration of GA3 for potatoes

is required before further recommendations can be made.

The concentration of GA3 applied was crucial for determining tuber numbers and yield

responses in potato. The appropriate concentration depended on variety. A

concentration of 20 mg/L was the optimum to increase tuber number and yield of 20-55

g tubers for Atlantic. Although 20 mg GA3/L resulted in the highest tuber number in

Granola, some tubers bent and elongated therefore lower concentrations is probably

suitable. Higher GA3 concentrations reduced tuber number and yield.

The method of GA3 application is important for determining its effectiveness (Marinus

and Bodlaender 1978; Bodlaender and van de Waart 1989; Struik et al. 1989b). Dipping

cut seeds was most effective for increasing stem and tuber number because GA was

readily absorbed by cut surface of the seeds (Holmes et al. 1970; Sekhon and Singh

1984). Compared with other methods, such as foliar sprays (Bodlaender and van de

Waart 1989) the dipping method was more convenient and more practical because,

unlike spraying, it was not dependent on weather conditions. Sprays cannot be applied

during windy or wet conditions. The dipping method is probably the easiest for growers

to adopt.

The timing of GA3 application was also critical in determining the plant response.

Gibberellin application to seed pieces prior to planting stimulated bud, and sprout

growth rate and increased the number of stems developed. Gibberellin application is

known to inhibit tuber initiation (Menzel 1980; Vreugdenhil and Struik 1989; Abdala et

al. 1995) thus it needs to be applied well before the tuber initiation stage. Giving seed a

pre-planting dip in GA3 solution not only allows it to be absorbed and translocated to

General discussion

128

buds where it promotes multiple stem growth but also provides a sufficient time interval

to ensure GA3 is metabolized into inactive forms before tuber initiation begins (Reeve

and Crozier 1974; Lenton and Appleford 1991).

It was found that, amongst the techniques studied, treating seed with GA3 was the most

successful treatment for increasing stem number, tuber number and shifting yield

toward a greater proportion of small tubers. However, in order to make this technique

available to seed farmers we need to register GA3. Factors that support the registration

of GA3 for seed potato production include the findings that it has no carry over effects

on subsequent crop production (Bishop and Timm 1968; Holmes et al. 1970;

Bodlaender and van de Waart 1989). Furthermore, the GA treatments are applied on

seeds that are a generation removed from harvest and so it should not be dangerous for

human consumption. The fact that GA3 naturally occurs in potato plants (Xu et al.

1998b; Berg et al. 1995; Abdala et al. 2002) and that it has been used for production of

table grapes and oranges in Australia should further ease the registration process.

In my study that examined the potential use of combining GA3 and paclobutrazol, I

found that it reduced total tuber number and total yield compared with GA3 alone. This

was probably because the timing of paclobutrazol application was not appropriate.

The plants grown in Manjimup yielded twice that of plants grown in Perth. This was

found to relate to the length of growing period, irrigation rate and site characteristics.

Amongst these, the length of the growing period, which was influenced by temperature

and photoperiod (Kooman et al. 1996a; Kooman and Rabbinge 1996) was the most

important factor determining yield. The length of growing period was 118 DAP in Perth

and 146 DAP in Manjimup. Tuber dry matter production is significantly influenced by

the length of growing period. Longer growing periods result in higher yield due to

longer ground-cover duration and greater radiation interception (Kooman et al. 1996b).

High temperatures cause early senescence (Menzel 1985) particularly temperatures

General discussion

129

above 30oC (Fahem and Haverkort 1988; Midmore 1990). In Perth, in December, the

mean maximum temperatures were greater than 30oC and are probably responsible for

early plant senesce and thus a shorter growing period. Early senescence shortened the

time interval that plants intercepted photosynthetically active radiation (PAR) and

reduced the photosynthetic capacity and thus yield. High temperatures are also known

to reduced yield through their influence on assimilate partitioning and dry matter

production, which favors shoot growth over tuber growth (Wolf et al. 1990). The

efficiency in converting intercepted solar radiation to tuber dry matter reduces at high

temperatures. In Manjimup, the temperature ranged between 16 and 26oC across the

entire growing season and helped prolong the growing period resulting in higher yield

than that in Perth.

The length of growing period is also influenced by photoperiod. Generally long days

prolong the life of foliage (Burton 1989) and promote shoot growth whilst the reverse is

true for short days (Beukema and van der Zaag 1990). Particularly, short-day length

during shoot emergence reduces the growth period (Kooman et al. 1996a). Potatoes

planted in Manjimup in mid-spring and growth through summer experienced longer

days than those grown in Perth during late winter through spring. Therefore, potatoes in

Manjimup had a longer growing period than Perth.

In addition to the all of the above, potatoes grown in Perth also experienced water

deficit, early in the growing period, and this probably contributed to lower yield.

Untreated plants in Perth yielded about 39 t/ha and this was slightly low compared to 45

t/ha in other experiments with similar planting and harvesting dates on the Swan Coastal

plain of Western Australia (McPharlin and Dawson 1998). Also the average yield of

potatoes planted in sandy soil in Western Australia is 45 t/ha (Rogger State,

pers.comm.). Under water deficit stomata tend to close (Epstein and Grant 1973) and in

potato lead to decreased gas exchange and photosynthetic rates (Chapman and Loomis

General discussion

130

1953; Shekhar and Iritani 1979) thus reducing tuber dry matter production and yield

(Cavangnaro et al. 1971; Steckel and Gray 1979; Jefferies and Mackerron 1987a).

Potatoes grown in Manjimup were not limited by irrigation water, whereas the Perth site

was.

The site in Manjimup was ideal whilst Perth was a more difficult environment for

growing potatoes. Not only did Manjimup have cooler air temperatures but also higher

relative humidity compared with that in Perth. Potato is a cool climate crop (Ewing

1981; Haverkort 1990) and was therefore expected to grow better and yield higher in

Manjimup compared with Perth. Moreover, the soil properties such as N, P and K

content in Manjimup were more favourable than that in Perth. The Perth site had more

fertilizer applied at weekly intervals because more nutrients were required but they were

more likely to be leached from the sandy soil by rain or irrigation (McPharlin 2003).

The soil texture of sandy loam in Manjimup probably had a better water holding

capacity than the sand used in Perth. The low water holding capacity of sand soil can

cause water stress injury (Miller and Martin 1987) and daily irrigation to cover daily

evapotranspiration rate is required to maintain tuber yield (Martin and Miller 1983). The

two different soil textures required different irrigation management and sandy soils

required more frequent irrigation than sandy loams.

In conclusion, for the cv. Atlantic and Granola, neither carvone application nor low and

very-low rates of herbicide (i.e. paraquat + diquat) application resulted in increased

numbers of tubers or a greater yield of small round seeds that could be useful for small

seed potato production. Paclobutrazol is also not likely to be useful for seed production

in variety Atlantic and Granola because its influence on total tuber number, total yield

and yield of small round seeds was highly variable. Although apical sprout removal was

very effective for increasing stem number, tuber number, total yield and the yield of

General discussion

131

small tubers in Granola but not in Atlantic, it is a practical technique for farmers. A

combination of GA3 and paclobutrazol, reduced total tuber number and total yield.

The application of gibberellic acid was found to be the best method for reducing tuber

size in variety Atlantic which naturally produces few, large tubers. Application of GA

increased stem number, tuber number, and the yield of small tubers without reducing

the total yield in both Atlantic and Granola but further testing of lower GA3

concentrations are required in order to prevent the development of deformed tubers.

Thus, my investigation has resulted in the development of an effective and practical

method for large-scale production of small round Atlantic tubers, under the field

conditions in Western Australia. Once GA3 has been registered for seed potato

production in Western Australia the export of small round seed potatoes to Indonesia

may increase substantially.

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Documents

Abstract - the UWA Profiles and Research Repository · Mr Tom Fox for providing certified “Jasper Lake” seed potatoes cv. Atlantic and Granola. My parents, bapak Ketut Suwinda